vxlan: reuse inner packet flow hash for tunnel outer header load balance

vxlan: reuse inner packet flow hash for tunnel outer header load balance

Type: fix

Several tunnels encapsulation use udp as outer header and udp src port
is set by inner header flow hash, such as gtpu, geneve, vxlan, vxlan-gbd
Since flow hash of inner header is already been calculated, keeping it
to vnet_buffere[b]->ip.flow_hash should save load-balance node work to
select ECMP uplinks.

Change-Id: I0e4e2b27178f4fcc5785e221d6d1f3e8747d0d59
Signed-off-by: Shawn Ji <xiaji@tethrnet.com>