Add vnode_pager_purge_range(9) KPI

This KPI is created in addition to the existing vnode_pager_setsize(9)
KPI. The KPI is intended for file systems that are able to turn a range
of file into sparse range, also known as hole-punching.

Sponsored by: The FreeBSD Foundation
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D27194