ceph: don't WARN if we're still opening a session to an MDS

If MDSs aren't available while mounting a filesystem, the session state
will transition from SESSION_OPENING to SESSION_CLOSING.  And in that
scenario check_session_state() will be called from delayed_work() and
trigger this WARN.

Avoid this by only WARNing after a session has already been established
(i.e., the s_ttl will be different from 0).

Fixes: 62575e27 ("ceph: check session state after bumping session->s_seq")
Signed-off-by: NLuis Henriques <lhenriques@suse.de>
Reviewed-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -4456,7 +4456,7 @@ bool check_session_state(struct ceph_mds_session *s)
 		break;
 	case CEPH_MDS_SESSION_CLOSING:
 		/* Should never reach this when we're unmounting */
-		WARN_ON_ONCE(true);
+		WARN_ON_ONCE(s->s_ttl);
 		fallthrough;
 	case CEPH_MDS_SESSION_NEW:
 	case CEPH_MDS_SESSION_RESTARTING: