gnrc_sixlowpan_frag: unroll recursion
gnrc_sixlowpan_frag: unroll recursion

While the recursion in `gnrc_sixlowpan_frag` shouldn't be infinite we
still should avoid using recursions in general (also to be able to
statically analyze stack usage). This unrolls the recursion.

OlegHahm/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,smlng/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,OTAkeys/RIOT,basilfx/RIOT,authmillenon/RIOT,mfrey/RIOT,kaspar030/RIOT,kYc0o/RIOT,yogo1212/RIOT,jasonatran/RIOT,kaspar030/RIOT,lazytech-org/RIOT,yogo1212/RIOT,rfuentess/RIOT,basilfx/RIOT,BytesGalore/RIOT,mfrey/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,A-Paul/RIOT,rfuentess/RIOT,jasonatran/RIOT,authmillenon/RIOT,OlegHahm/RIOT,A-Paul/RIOT,x3ro/RIOT,mtausig/RIOT,mfrey/RIOT,cladmi/RIOT,authmillenon/RIOT,josephnoir/RIOT,A-Paul/RIOT,basilfx/RIOT,jasonatran/RIOT,kYc0o/RIOT,toonst/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,rfuentess/RIOT,yogo1212/RIOT,smlng/RIOT,kYc0o/RIOT,yogo1212/RIOT,rfuentess/RIOT,ant9000/RIOT,x3ro/RIOT,toonst/RIOT,smlng/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,smlng/RIOT,miri64/RIOT,lazytech-org/RIOT,ant9000/RIOT,miri64/RIOT,basilfx/RIOT,josephnoir/RIOT,BytesGalore/RIOT,OlegHahm/RIOT,ant9000/RIOT,lazytech-org/RIOT,RIOT-OS/RIOT,BytesGalore/RIOT,mtausig/RIOT,ant9000/RIOT,rfuentess/RIOT,cladmi/RIOT,x3ro/RIOT,cladmi/RIOT,aeneby/RIOT,BytesGalore/RIOT,yogo1212/RIOT,aeneby/RIOT,josephnoir/RIOT,mfrey/RIOT,yogo1212/RIOT,miri64/RIOT,josephnoir/RIOT,mtausig/RIOT,aeneby/RIOT,OTAkeys/RIOT,A-Paul/RIOT,toonst/RIOT,cladmi/RIOT,cladmi/RIOT,mtausig/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,mfrey/RIOT,x3ro/RIOT,smlng/RIOT,toonst/RIOT,x3ro/RIOT,OlegHahm/RIOT,aeneby/RIOT,aeneby/RIOT,toonst/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,mtausig/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,basilfx/RIOT