dtrace: third update to the siftr probe

This adds the changes corresponding to
https://cgit.freebsd.org/src/commit/?id=60167184abd54ca12c6bf7ab60f2a08b41342f84
to the siftr probe.
This concludes the series up updates.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40825