netfilter: flowtable: add offload support for xmit path types

When the flow tuple xmit_type is set to FLOW_OFFLOAD_XMIT_DIRECT, the
dst_cache pointer is not valid, and the h_source/h_dest/ifidx out fields
need to be used.

This patch also adds the FLOW_ACTION_VLAN_PUSH action to pass the VLAN
tag to the driver.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>