netfilter: ebtables: fix compat entry padding

On arm64, ebt_entry_{match,watcher,target} structs are 40 bytes long
while on 32-bit arm these structs have a size of 36 bytes.

COMPAT_XT_ALIGN() macro cannot be used here to determine the necessary
padding for the CONFIG_COMPAT because it imposes an 8-byte boundary
alignment, condition that is not found in 32-bit ebtables application.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/bridge/netfilter/ebtables.c

@@ -1610,16 +1610,16 @@ struct compat_ebt_entry_mwt {
 		compat_uptr_t ptr;
 	} u;
 	compat_uint_t match_size;
-	compat_uint_t data[0];
+	compat_uint_t data[0] __attribute__ ((aligned (__alignof__(struct compat_ebt_replace))));
 };
 
 /* account for possible padding between match_size and ->data */
 static int ebt_compat_entry_padsize(void)
 {
-	BUILD_BUG_ON(XT_ALIGN(sizeof(struct ebt_entry_match)) <
-			COMPAT_XT_ALIGN(sizeof(struct compat_ebt_entry_mwt)));
-	return (int) XT_ALIGN(sizeof(struct ebt_entry_match)) -
-			COMPAT_XT_ALIGN(sizeof(struct compat_ebt_entry_mwt));
+	BUILD_BUG_ON(sizeof(struct ebt_entry_match) <
+			sizeof(struct compat_ebt_entry_mwt));
+	return (int) sizeof(struct ebt_entry_match) -
+			sizeof(struct compat_ebt_entry_mwt);
 }
 
 static int ebt_compat_match_offset(const struct xt_match *match,