vm: implement vm_page_reclaim_contig_domain_ext()

Implement vm_page_reclaim_contig_domain_ext() to reclaim multiple
contiguous regions at once. This makes it more efficient for users
that need multiple contiguous regions to reclaim those regions
efficiently.

This is needed because callers like ktls may need to reclaim many
contiguous regions, and each scan of physical memory can take
multiple seconds on a large memory machine (order of 100GB of
RMA). Rather than modifying the core algorithm, I extended
vm_page_reclaim_contig_domain() to take a "desired_runs" argument to
allow the caller to request that it reclaim more than just a single
run. There is no functional change intended for all existing
callers.

The first user for this interface is the ktls code
(https://reviews.freebsd.org/D39421). By reclaiming multiple runs,
ktls goes from consuming hours of CPU to refill its buffer zone to
just seconds or minutes.

Differential Revision: https://reviews.freebsd.org/D39739
Sponsored by: Netflix
Reviewed by: alc, jhb, markj