dtrace: fix constants used by the siftr probe

The constants used in the C code were changed in
https://svnweb.freebsd.org/changeset/base/343646
but changing them also in the dtrace translator was missed.

Reviewed by: cc, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40805

(cherry picked from commit f5541b85a592da9e1e1723b190f1e08afd7d5b18)