Make rewrite use Uncached I/O

Rewrite is a one-time/rare bulk administrative operation, which
should minimally affect payload caching. Plus some avoided memory
copies in its data path allow to significantly increase its speed.

My tests show reduction of time to rewrite 28GB of uncompressed
files on NVMe pool from 17 to 9 seconds and minimal ARC usage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #17407