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
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40805