vxlan-gbp: prefetch encap header memory area
vxlan-gbp: prefetch encap header memory area

This patch helps save 4.1 clocks/pkt from 62.9 to 58.8
clocks/pkt on Skylake.

Change-Id: I749a88a8fa6c78243441a89d6afcd04f106af3da
Signed-off-by: Zhiyong Yang <fbdb847fac8e5e50b588eb93d9ff9fd811b7568c@intel.com>

FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp