netfilter: fix description of expected checkentry return code on xt_target

The text describing the return codes that are expected on calls to
checkentry() was incorrect.  Instead of returning true or false, or an error
code, it should return 0 or an error code.
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter/x_tables.h

@@ -333,7 +333,7 @@ struct xt_target {
 	/* Called when user tries to insert an entry of this type:
            hook_mask is a bitmask of hooks from which it can be
            called. */
-	/* Should return true or false, or an error code (-Exxxx). */
+	/* Should return 0 on success or an error code otherwise (-Exxxx). */
 	int (*checkentry)(const struct xt_tgchk_param *);
 
 	/* Called when entry of this type deleted. */