nfsd4: require version 4 when enabling or disabling minorversion

The current code will allow silly things like:

	echo "+2 +3 +4 +7.1">/proc/fs/nfsd/versions

Reported-by: Fan Chaoting <fanchaoting@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 7493428..65889ec 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -534,7 +534,7 @@
 			else
 				num = simple_strtol(vers, &minorp, 0);
 			if (*minorp == '.') {
-				if (num < 4)
+				if (num != 4)
 					return -EINVAL;
 				minor = simple_strtoul(minorp+1, NULL, 0);
 				if (minor == 0)