dtrace: get rid of uchar_t types

Callers are specifying uint8_t anyway and this slightly reduces
dependencies on compatibility typedefs. No functional change intended.

Reviewed by: markj, mhorne
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39490

(cherry picked from commit 1a149d65baed31b90a0b8abe2d6c5a568122a5ec)