Set RX sw_if_index on decrypted routed IPsec buffers

For routed IPsec, set the RX sw_if_index on inbound packets
to the index of the IPsec interface. When a packet is
decrypted into a new buffer, bring along the RX sw_if_index
of the encrypted packet to the new buffer.

Change-Id: I093e9d37def2082c8d2f1deb96b1c5b97126e023
Signed-off-by: Matthew Smith <mgsmith@netgate.com>