e1000e: allow tx of pre-formatted vlan tagged packets

As with igb, when the e1000e driver is fed 802.1q
packets with hardware checksum on, it chokes with an
error of the form:

checksum_partial 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 wiht 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.

Fixups from Stephen Hemminger <shemminger@vyatta.com> and
Alexander Duyck <alexander.h.duyck@intel.com>

Signed-off-by: Arthur Jones <ajones@riverbed.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>