dpdk: add esp6 decrypt nodes, func declarations

dpdk: add esp6 decrypt nodes, func declarations

The function dpdk_ipsec_process() attempts to initialize some
globals that store node indexes after looking up the node
dpdk-esp6-decrypt. No such node was declared, so a segv
occurs after dereferencing the result of the lookup.

Add a node function that invokes dpdk_esp_decrypt_inline()
with is_ip6 set to 1. Add a declaration of node dpdk-esp6-decrypt
that uses the node function.

Change-Id: I31ce23a458c2d4181bf40cbc2118c4ef3b9baf97
Signed-off-by: Matthew Smith <mgsmith@netgate.com>