examples/ipsec-secgw: update eth header during route lookup

examples/ipsec-secgw: update eth header during route lookup

Update ethernet header during route lookup instead of doing
way later while performing Tx burst. Advantages to doing
is at route lookup is that no additional IP version checks
based on packet data are needed and packet data is already
in cache as route lookup is already consuming that data.

This is also useful for inline protocol offload cases
of v4inv6 or v6inv4 outbound tunnel operations as
packet data will not have any info about what is the tunnel
protocol.

Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>