vxlan-gpe: improve encap performance

This patch improves performance by prefetching encap header area
and taking full advantage of optimized function vlib_get_buffers.
After applying the patch, the function vxlan_gpe_encap can save
4.1 clocks/pkt from 41.7 to 37.6 clocks/pkt on Skylake.

Change-Id: I85d486b21a2524d64f2e246dfb4183539ec2532d
Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>