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>