xtimer/xtimer.c: ensure modifing mutex is done with interrupt disabled
xtimer/xtimer.c: ensure modifing mutex is done with interrupt disabled

if xtimer_set spins the callback is executed in the thread context.

comment to explain irq_disable
and when this line could be removed
(when xtimer stops executing the callback funtion from thread context)

yogo1212/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OlegHahm/RIOT,x3ro/RIOT,mtausig/RIOT,OTAkeys/RIOT,aeneby/RIOT,ant9000/RIOT,miri64/RIOT,x3ro/RIOT,authmillenon/RIOT,aeneby/RIOT,smlng/RIOT,yogo1212/RIOT,smlng/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,smlng/RIOT,yogo1212/RIOT,josephnoir/RIOT,josephnoir/RIOT,smlng/RIOT,toonst/RIOT,jasonatran/RIOT,rfuentess/RIOT,kYc0o/RIOT,cladmi/RIOT,x3ro/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,x3ro/RIOT,yogo1212/RIOT,rfuentess/RIOT,OlegHahm/RIOT,toonst/RIOT,ant9000/RIOT,RIOT-OS/RIOT,mtausig/RIOT,cladmi/RIOT,basilfx/RIOT,OTAkeys/RIOT,mtausig/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,rfuentess/RIOT,kaspar030/RIOT,cladmi/RIOT,kaspar030/RIOT,josephnoir/RIOT,basilfx/RIOT,toonst/RIOT,jasonatran/RIOT,josephnoir/RIOT,toonst/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,rfuentess/RIOT,basilfx/RIOT,rfuentess/RIOT,smlng/RIOT,OTAkeys/RIOT,mtausig/RIOT,mtausig/RIOT,authmillenon/RIOT,aeneby/RIOT,jasonatran/RIOT,aeneby/RIOT,miri64/RIOT,aeneby/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,jasonatran/RIOT,toonst/RIOT,authmillenon/RIOT,cladmi/RIOT,OlegHahm/RIOT,yogo1212/RIOT,miri64/RIOT,basilfx/RIOT,basilfx/RIOT,jasonatran/RIOT,x3ro/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,cladmi/RIOT