siftr: unbreak dtrace support

This patch adds back some fields needed by the siftr probe, which were
removed in
https://cgit.freebsd.org/src/commit/?id=aa61cff4249c92689d7a1f15db49d65d082184cb

With this fix, you can run dtrace scripts again when the siftr
module is loaded. And the siftr probe works again.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40826