librbd: potential assertion failure during cache read

librbd: potential assertion failure during cache read

It's possible for a cache read from a clone to trigger a writeback if a
previous read op determined the object doesn't exist in the clone,
followed by a cached write to the non-existent clone object, followed
by another read request to the same object.  This causes the cache to
flush the pending writeback ops while not holding the owner lock.

Fixes: #13559
Backport: hammer
Signed-off-by: Jason Dillaman <dillaman@redhat.com>