Add BRT support to zpool prefetch command

Implement BRT (Block Reference Table) prefetch functionality similar
to existing DDT prefetch.  This allows preloading BRT metadata into
ARC to improve performance for block cloning operations and frees
of earlier cloned blocks.

Make -t parameter optional.  When omitted, prefetch all supported
metadata types (both DDT and BRT now).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17890

man/man8/zpool-prefetch.8

@@ -28,20 +28,25 @@
 .
 .Sh NAME
 .Nm zpool-prefetch
-.Nd Loads specific types of data for the given pool
+.Nd Prefetches pool metadata into ARC
 .Sh SYNOPSIS
 .Nm zpool
 .Cm prefetch
-.Fl t Ar type
+.Op Fl t Ar type
 .Ar pool
 .Sh DESCRIPTION
-.Bl -tag -width Ds
-.It Xo
-.Nm zpool
-.Cm prefetch
-.Fl t Li ddt
-.Ar pool
-.Xc
-Prefetch data of a specific type for the given pool; specifically the DDT,
-which will improve write I/O performance when the DDT is resident in the ARC.
+Massively prefetch metadata of a specific type for the given pool into the ARC
+to reduce latency of some operations later.
+If no type is specified, all types are prefetched.
+.Pp
+The following types are supported:
+.Bl -tag -width "brt"
+.It Sy brt
+Prefetch the BRT (block reference table).
+This may improve performance for block cloning operations,
+and frees for earlier cloned blocks.
+.It Sy ddt
+Prefetch the DDT (deduplication table).
+This may improve performance of writes when deduplication is enabled,
+and frees for earlier deduplicated blocks.
 .El