NFSD: Create PF_INET6 listener in write_ports

Try to create a PF_INET6 listener for NFSD, if IPv6 is enabled in the
kernel.

Make sure nfsd_serv's reference count is decreased if
__write_ports_addxprt() failed to create a listener.  See
__write_ports_addfd().

Our current plan is to rely on rpc.nfsd to create appropriate IPv6
listeners when server-side NFS/IPv6 support is desired.  Legacy
behavior, via the write_threads or write_svc kernel APIs, will remain
the same -- only IPv4 listeners are created.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
[bfields@citi.umich.edu: Move error-handling code to end]
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfsctl.c

@@ -988,6 +988,7 @@ static ssize_t __write_ports_delfd(char *buf)
 static ssize_t __write_ports_addxprt(char *buf)
 {
 	char transport[16];
+	struct svc_xprt *xprt;
 	int port, err;
 
 	if (sscanf(buf, "%15s %4u", transport, &port) != 2)
@@ -1003,8 +1004,23 @@ static ssize_t __write_ports_addxprt(char *buf)
 	err = svc_create_xprt(nfsd_serv, transport,
 				PF_INET, port, SVC_SOCK_ANONYMOUS);
 	if (err < 0)
-		return err;
+		goto out_err;
+
+	err = svc_create_xprt(nfsd_serv, transport,
+				PF_INET6, port, SVC_SOCK_ANONYMOUS);
+	if (err < 0 && err != -EAFNOSUPPORT)
+		goto out_close;
 	return 0;
+out_close:
+	xprt = svc_find_xprt(nfsd_serv, transport, PF_INET, port);
+	if (xprt != NULL) {
+		svc_close_xprt(xprt);
+		svc_xprt_put(xprt);
+	}
+out_err:
+	/* Decrease the count, but don't shut down the service */
+	nfsd_serv->sv_nrthreads--;
+	return err;
 }
 
 /*