offloading: Support multiple vlan tags in GSO.

We assume that hardware TSO can't support multiple levels of vlan tags
but we allow it to be done.  Therefore, enable GSO to parse these tags
so we can fallback to software.

Signed-off-by: Jesse Gross <jesse@nicira.com>
CC: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>