ksmbd: fix slab-use-after-free in ksmbd_smb2_session_create

There is a race condition between ksmbd_smb2_session_create and
ksmbd_expire_session. This patch add missing sessions_table_lock
while adding/deleting session from global session table.

Cc: stable@vger.kernel.org # v5.15+
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>