siftr: provide dtrace with the correct pointer to data

This fixes a bug which was introduced in the commit
https://svnweb.freebsd.org/changeset/base/282276

Reviewed by: cc, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40806

(cherry picked from commit dc2d26df43cb69c9c4d8802220af2da929bc1dce)