nfsd4: remove unnecessary comment

For the most part readers of cl_cb_state only need a value that is
"eventually" right.  And the value is set only either 1) in response to
some change of state, in which case it's set to UNKNOWN and then a
callback rpc is sent to probe the real state, or b) in the handling of a
response to such a callback.  UNKNOWN is therefore always a "temporary"
state, and for the other states we're happy to accept last writer wins.

So I think we're OK here.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -756,7 +756,6 @@ static void do_probe_callback(struct nfs4_client *clp)
  */
 void nfsd4_probe_callback(struct nfs4_client *clp)
 {
-	/* XXX: atomicity?  Also, should we be using cl_flags? */
 	clp->cl_cb_state = NFSD4_CB_UNKNOWN;
 	set_bit(NFSD4_CLIENT_CB_UPDATE, &clp->cl_flags);
 	do_probe_callback(clp);