Add vxlan-bypass feature to IP4 forwarding path

Add vxlan-bypass feature to IP4 forwarding path

Added vxlan-bypass feature which can be enabled on the underlay
interface which receive VXLAN packets to accelerate VXLAN decap
processing. The CLI to enable it is:
    set interface ip vxlan-bypass <interface> [del]
With this feature enabled on an interface, there is an overhead for
non-VXLAN packets at about 13 clocks per packet.
The API is created to support vxlan-bypass for both VXLANoIPv4 and
VXLANoIPv6. With this change, vxlan-bypass is only implemented for
IPv4 forwarding path.

Change-Id: Ica7239f642010b231eade7fd5009aa4b42c119d3
Signed-off-by: John Lo <loj@cisco.com>