CVE-2011-3593 kernel: vlan: fix panic when handling priority tagged frames

[Eugene Teo <eugene () redhat com>]

A partner reported that frames with priority tags only (VID=0 in the
frame), could cause a panic on some drivers. It affects the Linux kernel
as shipped with Red Hat Enterprise Linux 6 due to incorrect backporting
of upstream patches. It does not affect the upstream kernel.

https://bugzilla.redhat.com/CVE-2011-3593

Thanks, Eugene