igb: allow tx of pre-formatted vlan tagged packets

When the 82575 is fed 802.1q packets, it chokes with
an error of the form:

igb 0000:08:00.1 partial checksum but proto=81!

As the logic there was not smart enough to look into
the vlan header to pick out the encapsulated protocol.

There are times when we'd like to send these packets
out without having to configure a vlan on the interface.
Here we check for the vlan tag and allow the packet to
go out with the correct hardware checksum.

Thanks to Kand Ly <kand@riverbed.com> for discovering the
issue and the coming up with a solution.  This patch is
based upon his work.

Signed-off-by: Arthur Jones <ajones@riverbed.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>