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

The IP Total Length field includes the IP header and data.  Make sure it
is valid and does not exceed the Ethernet payload size.

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>
