dtrace: update siftr probe

This adds the changes corresponding to
https://cgit.freebsd.org/src/commit/?id=1241e8e7aed5d709a6cf62024e9ae750981c03ae
to the siftr probe.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40823