rtl8139: check IP Header Length field (CVE-2015-5165)

The IP Header Length field was only checked in the IP checksum case, but
is used in other cases too.

Reported-by: 朱东海(启路) <donghai.zdh@alibaba-inc.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
