Protocol negotiation in mount.nfs does not correctly negotiate with a
server which only support NFSv3 and UDP.

When mount.nfs attempts an NFSv4 mount and fails with ECONNREFUSED
it does not fall back to NFSv3, as this is not recognised as a
"does not support NFSv4" error.
However ECONNREFUSED is a clear indication that the server doesn't
support TCP, and ipso facto does not support NFSv4.
So ECONNREFUSED should trigger a fallback from v4 to v2/3.

Once we allow that error, NFSv3 is attempted and mount.nfs talks to
rpcbind and discovers that UDP should be used for v3 and the mount
succeeds.

Signed-off-by: NeilBrown <neilb@suse.de>
Reported-by: Carsten Ziepke <kieltux@gmail.com>

diff --git a/utils/mount/stropts.c b/utils/mount/stropts.c
index a642394d2f5a..6d4fd70b7b9e 100644
--- a/utils/mount/stropts.c
+++ b/utils/mount/stropts.c
@@ -807,6 +807,9 @@ static int nfs_autonegotiate(struct nfsmount_info *mi)
 		/* Linux servers prior to 2.6.25 may return
 		 * EPERM when NFS version 4 is not supported. */
 		goto fall_back;
+	case ECONNREFUSED:
+		/* UDP-Only servers won't support v4 */
+		goto fall_back;
 	default:
 		return result;
 	}