ceph: don't blindly unregister session that is in opening state

handle_cap_export() may add placeholder caps to session that is in
opening state. These caps' session pointer become wild after session get
unregistered.

The fix is not to unregister session in opening state during mds failovers,
just let client to reconnect later when mds is recovered.

Link: https://tracker.ceph.com/issues/40190


Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>