Revert "FreeBSD: zfs_putpages: don't undirty pages until after write completes"

This causes async putpages to leave the pages sbusied for a long time,
which hurts concurrency. Revert for now until we have a better
approach.

This reverts commit 238eab7dc16932edbe9bcc990e8e5376bfe5b2ba.

Reported by: Ihor Antonov <ngor@hugpoint.tech>
Discussed with: Rob Norris <rob.norris@klarasystems.com>