Add linux mlslabel support

The ZFS update to onnv_141 brought with it support for a
security label attribute called mlslabel.  This feature
depends on zones to work correctly and thus I am disabling
it under Linux.  Equivilant functionality could be added
at some point in the future.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zfs_ioctl.c

@@ -384,6 +384,7 @@ zfs_secpolicy_write_perms_ds(const char *name, dsl_dataset_t *ds,
 static int
 zfs_set_slabel_policy(const char *name, char *strval, cred_t *cr)
 {
+#ifdef HAVE_MLSLABEL
 	char		ds_hexsl[MAXNAMELEN];
 	bslabel_t	ds_sl, new_sl;
 	boolean_t	new_default = FALSE;
@@ -471,6 +472,9 @@ out_check:
 	if (needed_priv != -1)
 		return (PRIV_POLICY(cr, needed_priv, B_FALSE, EPERM, NULL));
 	return (0);
+#else
+	return ENOTSUP;
+#endif /* HAVE_MLSLABEL */
 }
 
 static int