datapath: backport: Fix vlan_insert_tag_set_proto().

datapath: backport: Fix vlan_insert_tag_set_proto().

Fix cvlan test failure on old kernel versions with 802.1ad. The root
cause is the upcall re-inserts the VLAN back into the raw packet data,
but the TPID is hard coded to 0x8100. This affects kernels for which
HAVE_VLAN_INSERT_TAG_SET_PROTO is not set.

The below patch allows the cvlan and 802.ad tests to pass on debian
with 3.16 kernel.

Signed-off-by: Eric Garver <e@erig.me>
Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Acked-by: Eric Garver <e@erig.me>
Signed-off-by: Joe Stringer <joe@ovn.org>