ipsec: fix trace of GRE_teb packets

ipsec: fix trace of GRE_teb packets

The issue is not easily hit. When GRE_teb packets are received the post
crypto processing adjusts the l2.l2_len value in the vnet_buffer opaque
data. This is overwriting the ipsec opaque data. Later the trace code
fetches the sa_index from the ipsec opaque data. It's just an accident
that this currently works, if the ipsec data is changed so that the
sa_index moves around it will be overwritten by the l2_len modification.
Indeed, this was found b/c local development changes had moved the
sa_index so it was over-lapping with the l2_len memory space, and the UT
failed.

Type: fix
Change-Id: Iaecfa750cf0b36653fd9e75b4d799f323a14d932
Signed-off-by: Christian Hopps <chopps@labn.net>