ceph: replace hold_mutex flag with goto

ceph: replace hold_mutex flag with goto

All of the early exit paths need to drop the mutex; it is only the normal
path through the function that does not.  Skip the unlock in that case
with a goto out_unlocked.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Jianpeng Ma <majianpeng@gmail.com>