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: NFan Chaoting <fanchaoting@cn.fujitsu.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsctl.c

@@ -534,7 +534,7 @@ static ssize_t __write_versions(struct file *file, char *buf, size_t size)
 			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)