NFSD: Prevent NULL dereference in nfsd4_process_cb_update()

@SES is initialized to NULL. If __nfsd4_find_backchannel() finds no
available backchannel session, setup_callback_client() will try to
dereference @SES and segfault.

Fixes: dcbeaa6 ("nfsd4: allow backchannel recovery")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4callback.c

@@ -1464,6 +1464,8 @@ static void nfsd4_process_cb_update(struct nfsd4_callback *cb)
 		ses = c->cn_session;
 	}
 	spin_unlock(&clp->cl_lock);
+	if (!c)
+		return;
 
 	err = setup_callback_client(clp, &conn, ses);
 	if (err) {