rbd: don't assume rbd_is_lock_owner() for exclusive mappings

Expanding on the previous commit, assuming that rbd_is_lock_owner()
always returns true (i.e. that we are either in RBD_LOCK_STATE_LOCKED
or RBD_LOCK_STATE_QUIESCING) if the mapping is exclusive is wrong too.
In case ceph_cls_set_cookie() fails, the lock would be temporarily
released even if the mapping is exclusive, meaning that we can end up
even in RBD_LOCK_STATE_UNLOCKED.

IOW, exclusive mappings are really "just" about disabling automatic
lock transitions (as documented in the man page), not about grabbing
the lock and holding on to it whatever it takes.

Cc: stable@vger.kernel.org
Fixes: 637cd060

 ("rbd: new exclusive lock wait/wake code")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>