Correct IPSec SA statistic keeping

The IPsec SA statistic keeping is used even for decision making on expiry/rekeying SAs.
When there are multiple transformations being done the statistic keeping might be wrong.

This mostly impacts multiple encapsulations on IPsec since the usual scenario it is not noticed due to the code path not taken.

Differential Revision: https://reviews.freebsd.org/D3239
Reviewed by: ae, gnn
Approved by: gnn(mentor)