ceph: keep the session state until it is released

When reconnecting the session but if it is denied by the MDS due
to client was in blacklist or something else, kclient will receive
a session close reply, and we will never see the important log:

"ceph:  mds%d reconnect denied"

And with the confusing log:

"ceph:  handle_session mds0 close 0000000085804730 state ??? seq 0"

Let's keep the session state until its memories is released.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>