dtrace: Use size_t instead of uintptr_t to represent buffer offsets

This eases porting of DTrace to CHERI, where uintptr_t and size_t aren't
interchangeable.

No functional change intended.

Reviewed by: Domagoj Stolfa <domagoj.stolfa@gmail.com>
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D48625