disk: disk_access: changed mutex for a spinlock

disk_access functions were called from an interupt context, thus
crashing on mutexes (no sleep is allowed in interrupt context).
The usage of a spinlock is permitted. The functions guarded by
the spinlock are executed very quickly anyway, so the spinlock
is applicable.

Signed-off-by: Alexi Demers <alexi.demers@axceta.com>