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>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
datapath: Mark compatible with kernels up to 3.18.x

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Pravin B Shelar <pshelar@nicira.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>
Acked-by: Pravin B Shelar <pshelar@nicira.com>