Truncate HMAC output only if requested by the client

The output of HMAC was previously truncated to 12 bytes. This was only
correct in case of one particular crypto client - the new version of IPSEC.
Fix by taking into account the cri_mlen field in cryptoini session request
filled in by the client.

Submitted by: Michal Stanek <mst@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D6218