Move mlslabel hunk to new linux-have-mlslabel

Simply relocate this hunk with a little tweak to the new
linux-have-mlslabel topic branch which has been created to
disable this support on linux.

module/zfs/zfs_ioctl.c

@@ -339,7 +339,6 @@ zfs_secpolicy_write_perms(const char *name, const char *perm, cred_t *cr)
 static int
 zfs_set_slabel_policy(const char *name, char *strval, cred_t *cr)
 {
-#if 0
 	char		ds_hexsl[MAXNAMELEN];
 	bslabel_t	ds_sl, new_sl;
 	boolean_t	new_default = FALSE;
@@ -427,9 +426,6 @@ out_check:
 	if (needed_priv != -1)
 		return (PRIV_POLICY(cr, needed_priv, B_FALSE, EPERM, NULL));
 	return (0);
-#else
-	return EPERM;
-#endif
 }
 
 static int