netfilter: flowtable: Add FLOW_OFFLOAD_XMIT_UNSPEC xmit type

It could be xmit type was not set and would default to FLOW_OFFLOAD_XMIT_NEIGH
and in this type the gc expect to have a route info.
Fix that by adding FLOW_OFFLOAD_XMIT_UNSPEC which defaults to 0.

Fixes: 8b9229d1 ("netfilter: flowtable: dst_check() from garbage collector path")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>