offloading: Make scatter/gather more tolerant of vlans.

When checking if it is necessary to linearize a packet, we currently
use vlan_features if the packet contains either an in-band or out-
of-band vlan tag.  However, in-band tags aren't special in any way
for scatter/gather since they are part of the packet buffer and are
simply more data to DMA.  Therefore, only use vlan_features for out-
of-band tags, which could potentially have some interaction with
scatter/gather.

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