ZIL: Reduce maximum size of WR_COPIED to 7.5K

Benchmarks show that at certain write sizes range lock/unlock take
not so much time as extra memory copy.  The exact threshold is not
obvious due to other overheads, but it is definitely lower than
~63KB used before.  Make it configurable, defaulting at 7.5KB,
that is 8KB of nearest malloc() size minus itx and lr structs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15353

man/man4/zfs.4

@@ -2150,6 +2150,11 @@ On very fragmented pools, lowering this
 .Pq typically to Sy 36 KiB
 can improve performance.
 .
+.It Sy zil_maxcopied Ns = Ns Sy 7680 Ns B Po 7.5 KiB Pc Pq uint
+This sets the maximum number of write bytes logged via WR_COPIED.
+It tunes a tradeoff between additional memory copy and possibly worse log
+space efficiency vs additional range lock/unlock.
+.
 .It Sy zil_min_commit_timeout Ns = Ns Sy 5000 Pq u64
 This sets the minimum delay in nanoseconds ZIL care to delay block commit,
 waiting for more records.