netfilter: xtables: change xt_match.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.

This semantic patch may not be too precise (checking for functions
that use xt_mtchk_param rather than functions referenced by
xt_match.checkentry), but reviewed, it produced the intended result.

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

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

include/linux/netfilter/x_tables.h

@@ -303,7 +303,7 @@ struct xt_match {
 		      const struct xt_match_param *);
 
 	/* Called when user tries to insert an entry of this type. */
-	bool (*checkentry)(const struct xt_mtchk_param *);
+	int (*checkentry)(const struct xt_mtchk_param *);
 
 	/* Called when entry of this type deleted. */
 	void (*destroy)(const struct xt_mtdtor_param *);