BRT: Increase block size from 4KB to 8KB

According to my observations, BRT ZAPs are typically compressible
3:1 for data and 2:1 for indirects.  With ashift=12, typical these
days, it means increasing the block sizes to 8KB we may get most
of possible compression, reducing on-disk and in-ARC BRT footprint
in half by the cost of some compression/decompression overhead,
but without real write inflation, only some dirty data increase.

Increase to 32KB similar to DDT could further increase compression
and storage efficiency, but at the cost of write inflation and
much bigger dirty data increase, which we can not properly control
now.  So lets leave this for a time when BRT log gets implemented.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17916

man/man4/zfs.4

@@ -273,12 +273,12 @@ force this many of them to be gang blocks.
 .It Sy brt_zap_prefetch Ns = Ns Sy 1 Ns | Ns 0 Pq int
 Controls prefetching BRT records for blocks which are going to be cloned.
 .
-.It Sy brt_zap_default_bs Ns = Ns Sy 12 Po 4 KiB Pc Pq int
+.It Sy brt_zap_default_bs Ns = Ns Sy 13 Po 8 KiB Pc Pq int
 Default BRT ZAP data block size as a power of 2. Note that changing this after
 creating a BRT on the pool will not affect existing BRTs, only newly created
 ones.
 .
-.It Sy brt_zap_default_ibs Ns = Ns Sy 12 Po 4 KiB Pc Pq int
+.It Sy brt_zap_default_ibs Ns = Ns Sy 13 Po 8 KiB Pc Pq int
 Default BRT ZAP indirect block size as a power of 2. Note that changing this
 after creating a BRT on the pool will not affect existing BRTs, only newly
 created ones.

module/zfs/brt.c

@@ -260,8 +260,8 @@ static int brt_zap_prefetch = 1;
 #define	BRT_DEBUG(...)	do { } while (0)
 #endif
 
-static int brt_zap_default_bs = 12;
-static int brt_zap_default_ibs = 12;
+static int brt_zap_default_bs = 13;
+static int brt_zap_default_ibs = 13;
 
 static kstat_t	*brt_ksp;