ksmbd: fix encryption failure issue for session logoff response

If client send encrypted session logoff request on seal mount,
Encryption for that response fails.

ksmbd: Could not get encryption key
CIFS: VFS: cifs_put_smb_ses: Session Logoff failure rc=-512

Session lookup fails in ksmbd_get_encryption_key() because sess->state is
set to SMB2_SESSION_EXPIRED in session logoff. There is no need to do
session lookup again to encrypt the response. This patch change to use
ksmbd_session in ksmbd_work.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>