VPP-521: Classify API enhancement to redirect traffic to pre-defined VRF

VPP-521: Classify API enhancement to redirect traffic to pre-defined VRF

Ingress packets are punted to the “Input ACL node” where traffic is
classified based on n-tuple keys. If no matched session is found from
the classify tables, then it will be passed to “the lookup node” for
normal packet forwarding. If a classify session is hit from one of
classify tables, then packet vnet buffer field sw_if_index[VLIB_TX]
will be updated to the new FIB index used for subsequent IP lookup
for this packet.

Change-Id: Ifdea63196ddb81c2d5c43b8c98e11ddbf5b11858
Signed-off-by: Steve Shin <jonshin@cisco.com>