netfilter: xtables: change xt_target.checkentry return type

Restore function signatures from bool to int so that we can report
memory allocation failures or similar using -ENOMEM rather than
always having to pass -EINVAL back.

// <smpl>
@@
type bool;
identifier check, par;
@@
-bool check
+int check
 (struct xt_tgchk_param *par) { ... }
// </smpl>

Minus the change it does to xt_ct_find_proto.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

include/linux/netfilter/x_tables.h

@@ -342,8 +342,8 @@ 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. */
-	bool (*checkentry)(const struct xt_tgchk_param *);
+	/* Should return true or false, or an error code (-Exxxx). */
+	int (*checkentry)(const struct xt_tgchk_param *);
 
 	/* Called when entry of this type deleted. */
 	void (*destroy)(const struct xt_tgdtor_param *);