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

include/sys/fs/zfs.h

@@ -1713,7 +1713,8 @@ typedef enum {
 
 typedef enum {
 	ZPOOL_PREFETCH_NONE = 0,
-	ZPOOL_PREFETCH_DDT
+	ZPOOL_PREFETCH_DDT,
+	ZPOOL_PREFETCH_BRT
 } zpool_prefetch_type_t;
 
 typedef enum {