[NETFILTER]: ebtables: don't compute gap before checking struct type

We cannot compute the gap until we know we have a 'struct ebt_entry' and
not 'struct ebt_entries'. Failure to check can cause crash.

Tested-by: Santiago Garcia Mantinan <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Signed-off-by: Chuck Ebbert <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/netfilter/ebtables.c

···
 	struct ebt_entry_target *t;
 	struct ebt_target *target;
 	unsigned int i, j, hook = 0, hookmask = 0;
-	size_t gap = e->next_offset - e->target_offset;
+	size_t gap;
 	int ret;
 
 	/* don't mess with the struct ebt_entries */
···
 	if (ret != 0)
 		goto cleanup_watchers;
 	t = (struct ebt_entry_target *)(((char *)e) + e->target_offset);
+	gap = e->next_offset - e->target_offset;
 	target = find_target_lock(t->u.name, &ret, &ebt_mutex);
 	if (!target)
 		goto cleanup_watchers;