Increase the default vm.max_user_wired value.

Since r347532 (merged to stable/12) we only count user-wired pages
towards the system limit. However, we now also treat pages wired by
hypervisors (bhyve and virtualbox) as user-wired, so starting VMs with
large amounts of RAM tends to fail due to the low limit.

The purpose of the limit is to provide a seatbelt, not to impose some
policy on the use of wired memory. Thus, increase the default limit to
allow reasonable VM configurations to work without tuning.

Reviewed by: kib
Discussed with: dougm
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26424