datapath: Check if nla_is_last() is available in <net/netlink.h>

nla_is_last() is not available in 3.18, it's only in net-next.
Convert to grep based to check to account for distribution backports.

Fixes: 684b5f ("datapath: Rename last_action() as nla_is_last() and move to netlink.h")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath: Mark compatible with kernels up to 3.18.x

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath: Account for rename to vlan_insert_tag_set_proto()

__vlan_put_tag() was renamed to vlan_insert_tag_set_proto() with
the argument list kept intact.

Relates-to: 62749e ("vlan: rename __vlan_put_tag to vlan_insert_tag_set_proto")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
datapath: Account for already defined NETIF_F_GSO_ENCAP_ALL

Relates-to: f6eec614 ("openvswitch: Enable tunnel GSO for OVS bridge.")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>