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: David Miller <davem@davemloft.net> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by: David S. Miller <davem@davemloft.net>