dtrace: use dtrace_instr_size() in the riscv dtrace_subr.c
No functional change intended.
Reviewed by: mhorne, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39652
(cherry picked from commit 75081b9ed8e67c8443551828f1b5ccf3d449b41b)