hwpmc: use kstack_contains()

This existing helper function is preferable to the hand-rolled
calculation of the kstack bounds.

Make some small style improvements while here. Notably, rename every
instance of "r", the return address, to "ra". Tidy the includes in the
affected files.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39909