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: Luis Henriques <lhenriques@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>