ksmbd: fix use-after-free in ksmbd_session_rpc_open

A UAF issue can occur due to a race condition between
ksmbd_session_rpc_open() and __session_rpc_close().
Add rpc_lock to the session to protect it.

Cc: stable@vger.kernel.org
Reported-by: Norbert Szetei <norbert@doyensec.com>
Tested-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>