rbuf: does not remove oldest entry if we have entry for current fragment
rbuf: does not remove oldest entry if we have entry for current fragment

If all rbuf slots are in use, `_rbuf_gc` removes the oldest entry even if the
entry for the current fragment exists. This effectively decreases usable slots
by one. This patch makes `_rbuf_gc` removes the oldest entry only if there is
no entry for the current fragment.

aeneby/RIOT,OTAkeys/RIOT,gautric/RIOT,immesys/RiSyn,OTAkeys/RIOT,A-Paul/RIOT,rousselk/RIOT,rajma996/RIOT,tfar/RIOT,rousselk/RIOT,gebart/RIOT,rajma996/RIOT,gautric/RIOT,miri64/RIOT,dailab/RIOT,ks156/RIOT,jbeyerstedt/RIOT-OTA-update,plushvoxel/RIOT,immesys/RiSyn,kaleb-himes/RIOT,thomaseichinger/RIOT,kaspar030/RIOT,Josar/RIOT,TobiasFredersdorf/RIOT,LudwigKnuepfer/RIOT,jfischer-phytec-iot/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,shady33/RIOT,beurdouche/RIOT,Josar/RIOT,d00616/RIOT,shady33/RIOT,jremmert-phytec-iot/RIOT,neiljay/RIOT,OlegHahm/RIOT,jremmert-phytec-iot/RIOT,kaspar030/RIOT,yogo1212/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,syin2/RIOT,Ell-i/RIOT,MohmadAyman/RIOT,Josar/RIOT,RIOT-OS/RIOT,syin2/RIOT,yogo1212/RIOT,LudwigOrtmann/RIOT,zhuoshuguo/RIOT,basilfx/RIOT,jasonatran/RIOT,BytesGalore/RIOT,attdona/RIOT,jasonatran/RIOT,asanka-code/RIOT,BytesGalore/RIOT,miri64/RIOT,MohmadAyman/RIOT,gebart/RIOT,plushvoxel/RIOT,mtausig/RIOT,RIOT-OS/RIOT,ant9000/RIOT,neumodisch/RIOT,malosek/RIOT,rakendrathapa/RIOT,zhuoshuguo/RIOT,kbumsik/RIOT,dailab/RIOT,roberthartung/RIOT,kbumsik/RIOT,rajma996/RIOT,jfischer-phytec-iot/RIOT,asanka-code/RIOT,beurdouche/RIOT,malosek/RIOT,biboc/RIOT,yogo1212/RIOT,rajma996/RIOT,rajma996/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,BytesGalore/RIOT,asanka-code/RIOT,toonst/RIOT,ant9000/RIOT,wentaoshang/RIOT,backenklee/RIOT,aeneby/RIOT,katezilla/RIOT,OTAkeys/RIOT,smlng/RIOT,basilfx/RIOT,MohmadAyman/RIOT,hamilton-mote/RIOT-OS,hamilton-mote/RIOT-OS,malosek/RIOT,syin2/RIOT,d00616/RIOT,tfar/RIOT,x3ro/RIOT,kYc0o/RIOT,attdona/RIOT,alignan/RIOT,ks156/RIOT,kYc0o/RIOT,d00616/RIOT,biboc/RIOT,jfischer-phytec-iot/RIOT,roberthartung/RIOT,adjih/RIOT,mtausig/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,altairpearl/RIOT,adrianghc/RIOT,jfischer-phytec-iot/RIOT,mfrey/RIOT,yogo1212/RIOT,rfuentess/RIOT,gebart/RIOT,neumodisch/RIOT,jremmert-phytec-iot/RIOT,dkm/RIOT,Yonezawa-T2/RIOT,kaleb-himes/RIOT,roberthartung/RIOT,FrancescoErmini/RIOT,rakendrathapa/RIOT,BytesGalore/RIOT,neiljay/RIOT,dailab/RIOT,josephnoir/RIOT,cladmi/RIOT,LudwigOrtmann/RIOT,dkm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,Hyungsin/RIOT-OS,brettswann/RIOT,Ell-i/RIOT,ks156/RIOT,immesys/RiSyn,josephnoir/RIOT,kerneltask/RIOT,altairpearl/RIOT,x3ro/RIOT,kb2ma/RIOT,plushvoxel/RIOT,LudwigKnuepfer/RIOT,malosek/RIOT,attdona/RIOT,A-Paul/RIOT,jfischer-phytec-iot/RIOT,cladmi/RIOT,kb2ma/RIOT,x3ro/RIOT,MohmadAyman/RIOT,basilfx/RIOT,adjih/RIOT,katezilla/RIOT,mtausig/RIOT,hamilton-mote/RIOT-OS,gebart/RIOT,wentaoshang/RIOT,beurdouche/RIOT,MohmadAyman/RIOT,yogo1212/RIOT,zhuoshuguo/RIOT,OlegHahm/RIOT,d00616/RIOT,RIOT-OS/RIOT,backenklee/RIOT,mtausig/RIOT,avmelnikoff/RIOT,smlng/RIOT,syin2/RIOT,cladmi/RIOT,alignan/RIOT,rousselk/RIOT,Hyungsin/RIOT-OS,backenklee/RIOT,kerneltask/RIOT,kerneltask/RIOT,kaleb-himes/RIOT,brettswann/RIOT,shady33/RIOT,ant9000/RIOT,Hyungsin/RIOT-OS,josephnoir/RIOT,malosek/RIOT,rajma996/RIOT,miri64/RIOT,dailab/RIOT,Ell-i/RIOT,tfar/RIOT,adjih/RIOT,wentaoshang/RIOT,TobiasFredersdorf/RIOT,kerneltask/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,jbeyerstedt/RIOT-OTA-update,cladmi/RIOT,neiljay/RIOT,zhuoshuguo/RIOT,brettswann/RIOT,MohmadAyman/RIOT,jbeyerstedt/RIOT-OTA-update,immesys/RiSyn,OlegHahm/RIOT,authmillenon/RIOT,wentaoshang/RIOT,lazytech-org/RIOT,shady33/RIOT,miri64/RIOT,Yonezawa-T2/RIOT,thomaseichinger/RIOT,kbumsik/RIOT,adjih/RIOT,katezilla/RIOT,A-Paul/RIOT,authmillenon/RIOT,mfrey/RIOT,RIOT-OS/RIOT,backenklee/RIOT,smlng/RIOT,shady33/RIOT,tfar/RIOT,jremmert-phytec-iot/RIOT,lazytech-org/RIOT,jremmert-phytec-iot/RIOT,kaspar030/RIOT,toonst/RIOT,A-Paul/RIOT,kb2ma/RIOT,toonst/RIOT,FrancescoErmini/RIOT,backenklee/RIOT,kaleb-himes/RIOT,FrancescoErmini/RIOT,TobiasFredersdorf/RIOT,rakendrathapa/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,kb2ma/RIOT,adrianghc/RIOT,avmelnikoff/RIOT,ks156/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,beurdouche/RIOT,kerneltask/RIOT,plushvoxel/RIOT,FrancescoErmini/RIOT,mfrey/RIOT,rousselk/RIOT,brettswann/RIOT,kb2ma/RIOT,toonst/RIOT,immesys/RiSyn,hamilton-mote/RIOT-OS,rfuentess/RIOT,avmelnikoff/RIOT,asanka-code/RIOT,syin2/RIOT,x3ro/RIOT,aeneby/RIOT,aeneby/RIOT,Yonezawa-T2/RIOT,avmelnikoff/RIOT,rakendrathapa/RIOT,dkm/RIOT,FrancescoErmini/RIOT,neumodisch/RIOT,kYc0o/RIOT,altairpearl/RIOT,avmelnikoff/RIOT,LudwigOrtmann/RIOT,mfrey/RIOT,altairpearl/RIOT,ant9000/RIOT,LudwigKnuepfer/RIOT,smlng/RIOT,basilfx/RIOT,LudwigKnuepfer/RIOT,biboc/RIOT,beurdouche/RIOT,biboc/RIOT,shady33/RIOT,roberthartung/RIOT,adjih/RIOT,LudwigOrtmann/RIOT,thomaseichinger/RIOT,TobiasFredersdorf/RIOT,rakendrathapa/RIOT,LudwigKnuepfer/RIOT,brettswann/RIOT,katezilla/RIOT,A-Paul/RIOT,Yonezawa-T2/RIOT,alignan/RIOT,alignan/RIOT,rousselk/RIOT,asanka-code/RIOT,toonst/RIOT,malosek/RIOT,dkm/RIOT,gautric/RIOT,altairpearl/RIOT,Hyungsin/RIOT-OS,neiljay/RIOT,rfuentess/RIOT,TobiasFredersdorf/RIOT,attdona/RIOT,basilfx/RIOT,tfar/RIOT,wentaoshang/RIOT,Yonezawa-T2/RIOT,Yonezawa-T2/RIOT,miri64/RIOT,kaleb-himes/RIOT,wentaoshang/RIOT,Josar/RIOT,kaspar030/RIOT,josephnoir/RIOT,kYc0o/RIOT,thomaseichinger/RIOT,thomaseichinger/RIOT,asanka-code/RIOT,rousselk/RIOT,jremmert-phytec-iot/RIOT,neumodisch/RIOT,mfrey/RIOT,kbumsik/RIOT,Ell-i/RIOT,Hyungsin/RIOT-OS,immesys/RiSyn,josephnoir/RIOT,LudwigOrtmann/RIOT,neumodisch/RIOT,hamilton-mote/RIOT-OS,dailab/RIOT,d00616/RIOT,roberthartung/RIOT,smlng/RIOT,BytesGalore/RIOT,rakendrathapa/RIOT,zhuoshuguo/RIOT,d00616/RIOT,brettswann/RIOT,lazytech-org/RIOT,gautric/RIOT,mtausig/RIOT,jbeyerstedt/RIOT-OTA-update,attdona/RIOT,altairpearl/RIOT,ks156/RIOT,aeneby/RIOT,attdona/RIOT,alignan/RIOT,adrianghc/RIOT,cladmi/RIOT,authmillenon/RIOT,yogo1212/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,neumodisch/RIOT,rfuentess/RIOT,rfuentess/RIOT,lazytech-org/RIOT,biboc/RIOT,gautric/RIOT,kYc0o/RIOT,plushvoxel/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,katezilla/RIOT,neiljay/RIOT,gebart/RIOT,adrianghc/RIOT,zhuoshuguo/RIOT,jbeyerstedt/RIOT-OTA-update,Josar/RIOT,x3ro/RIOT,dkm/RIOT,adrianghc/RIOT,ant9000/RIOT,FrancescoErmini/RIOT,LudwigOrtmann/RIOT,kbumsik/RIOT,Ell-i/RIOT,authmillenon/RIOT