ubifs: Change value of mutex_is_locked()

The mutex lock and unlock functions are stubbed out and mutex_is_locked
was 0.  This caused asserts to fail in ubifs code when checking that the
mutex was locked.  For example,

UBIFS assert failed in ubifs_change_lp at 540
UBIFS assert failed in ubifs_release_lprops at 278

Assume that the "mutex" is locked since that is the normal case when it
is checked in the ubifs code.

Signed-off-by: Bradley Bolen <bradleybolen@gmail.com>