nfsd: return CLID_INUSE for unexpected SETCLIENTID_CONFIRM case

Somebody with a Solaris client was hitting this case.  We haven't
figured out why yet, and don't have a reproducer.  Meanwhile Frank
noticed that RFC 7530 actually recommends CLID_INUSE for this case.
Unlikely to help the original reporter, but may as well fix it.
Reported-by: Frank Filz <ffilzlnx@mindspring.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -3111,10 +3111,11 @@ nfsd4_setclientid_confirm(struct svc_rqst *rqstp,
 	/*
 	 * We try hard to give out unique clientid's, so if we get an
 	 * attempt to confirm the same clientid with a different cred,
-	 * there's a bug somewhere.  Let's charitably assume it's our
-	 * bug.
+	 * the client may be buggy; this should never happen.
+	 *
+	 * Nevertheless, RFC 7530 recommends INUSE for this case:
 	 */
-	status = nfserr_serverfault;
+	status = nfserr_clid_inuse;
 	if (unconf && !same_creds(&unconf->cl_cred, &rqstp->rq_cred))
 		goto out;
 	if (conf && !same_creds(&conf->cl_cred, &rqstp->rq_cred))