dtrace: another update of the siftr probe

This adds the changes corresponding to
https://cgit.freebsd.org/src/commit/?id=d090464ecd4af5cd400ef5cbbfe8409d019eac34
to the siftr probe.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40824