l2: fix vxlan src port entropy with mpls payload

l2: fix vxlan src port entropy with mpls payload

l2 tunnels like vxlan, gtpu, geneva use vnet_l2_compute_flow_hash() to
compute flow hash for udp src port entropy. In case of inner mpls tunnels
to the same lsr ethernet src and dst macs are the same, so l2 flow hash
is also the same leading to no src port entropy and the only rss queue
overflow on receiver side.

Fix it for all the possible vnet_l2_compute_flow_hash callers by making
mpls playload hash in additon to ip4/ip6. Visible performance impact is
not expected as it's only one check for mpls ethertype for common cases.

Type: fix
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: I69153d42fb3d7c094a670c674fac8d14039c626a