netlink: Remove netlink_compare_arg.trailer

Instead of computing the offset from trailer, this patch computes
netlink_compare_arg_len from the offset of portid and then adds 4
to it.  This allows trailer to be removed.
Reported-by: NDavid Miller <davem@davemloft.net>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -974,10 +974,11 @@ struct netlink_compare_arg
 {
 	possible_net_t pnet;
 	u32 portid;
-	char trailer[];
 };
 
-#define netlink_compare_arg_len offsetof(struct netlink_compare_arg, trailer)
+/* Doing sizeof directly may yield 4 extra bytes on 64-bit. */
+#define netlink_compare_arg_len \
+	(offsetof(struct netlink_compare_arg, portid) + sizeof(u32))
 
 static inline int netlink_compare(struct rhashtable_compare_arg *arg,
 				  const void *ptr)