librbd: properly handle potential object map failures

librbd: properly handle potential object map failures

Remove the "ceph_assert" statements and instead bubble any potential
error code up to the caller. The object map state machines should
attempt to return a 0 upon failure unless it was unable to flag the
object map as invalid.

Fixes: http://tracker.ceph.com/issues/36074
Signed-off-by: Jason Dillaman <dillaman@redhat.com>