rtl8139: avoid nested ifs in IP header parsing (CVE-2015-5165)

rtl8139: avoid nested ifs in IP header parsing (CVE-2015-5165)

Transmit offload needs to parse packet headers.  If header fields have
unexpected values the offload processing is skipped.

The code currently uses nested ifs because there is relatively little
input validation.  The next patches will add missing input validation
and a goto label is more appropriate to avoid deep if statement nesting.

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>
(cherry picked from commit 39b8e7dcaf04cbdb926b478f825b160d852752b5)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>