Update zfs_freebsd_need_inactive to fix mmapped writes

zfs_freebsd_need_inactive appears to been based on an unfinished
version of https://reviews.freebsd.org/D22130 which had a bug where
files written via mmap wouldn't actually persist.

Update the function to match the final version committed to FreeBSD.

Authored-by: Mateusz Guzik <mjg@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10527
Closes #10528