mds: notify clients if the session has already opened

mds: notify clients if the session has already opened

If the connection was accidently closed due to the socket issue or
something else the client will try to open the opened sessions, for
now the MDS will just discard the session open request.

But the client will keep waiting the reply from the mds forever.

We need to tell the clients what has happened instead of discard it
directly. And when the client get the session open reply, it can
do what needed.

Fixes: https://tracker.ceph.com/issues/53911
Signed-off-by: Xiubo Li <xiubli@redhat.com>