vxlan_gpe: optimize encap performance

The patch can reduce 13 cycles per packet for the graph node
vxlan-gpe-encap and increases 5% or so vxlan_gpe encap throughput
on Haswell platform for the best case (All pkts have the same
sw_if_index).

Change-Id: I9c70fd3e0f2f0a9d922cf64970d0b0d51b772024
Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>