locker: clear desc on unlock

Signed-off-by: Sage Weil <sage@inktank.com>
locker: set desc on lock

Signed-off-by: Sage Weil <sage@inktank.com>
lock: pass desc to lock operation; leave on unlock

Pass the desc to the lock operation.

The unlock operation now clears desc for us; no need to do it outselves.

Signed-off-by: Sage Weil <sage@inktank.com>