ddt: add support for prefetching tables into the ARC

This change adds a new `zpool prefetch -t ddt $pool` command which
causes a pool's DDT to be loaded into the ARC. The primary goal is to
remove the need to "warm" a pool's cache before deduplication stops
slowing write performance. It may also provide a way to reload portions
of a DDT if they have been flushed due to inactivity.

Sponsored-by: iXsystems, Inc.
Sponsored-by: Catalogics, Inc.
Sponsored-by: Klara, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Will Andrews <will.andrews@klarasystems.com>
Signed-off-by: Fred Weigel <fred.weigel@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Co-authored-by: Will Andrews <will.andrews@klarasystems.com>
Co-authored-by: Don Brady <don.brady@klarasystems.com>
Closes #15890

man/man8/zpool.8

@@ -26,7 +26,7 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd March 16, 2022
+.Dd February 14, 2024
 .Dt ZPOOL 8
 .Os
 .
@@ -168,6 +168,8 @@ specified.
 .
 .Ss Maintenance
 .Bl -tag -width Ds
+.It Xr zpool-prefetch 8
+Prefetches specific types of pool data.
 .It Xr zpool-scrub 8
 Begins a scrub or resumes a paused scrub.
 .It Xr zpool-checkpoint 8
@@ -598,6 +600,7 @@ don't wait.
 .Xr zpool-list 8 ,
 .Xr zpool-offline 8 ,
 .Xr zpool-online 8 ,
+.Xr zpool-prefetch 8 ,
 .Xr zpool-reguid 8 ,
 .Xr zpool-remove 8 ,
 .Xr zpool-reopen 8 ,