Bring per_txg_dirty_frees_percent back to 30

The current value causes significant artificial slowdown during mass
parallel file removal, which can be observed both on FreeBSD and Linux
when running real workloads.

Sample results from Linux doing make -j 96 clean after an allyesconfig
modules build:

before: 4.14s user 6.79s system 48% cpu 22.631 total
after:	4.17s user 6.44s system 153% cpu 6.927 total

FreeBSD results in the ticket.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Richard Yao <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by:	Mateusz Guzik <[email protected]>
Closes openzfs#13932
Closes openzfs#13938

Author: mjguzik

man/man4/zfs.4

@@ -1626,7 +1626,7 @@ prefetched during a pool traversal, like
 .Nm zfs Cm send
 or other data crawling operations.
 .
-.It Sy zfs_per_txg_dirty_frees_percent Ns = Ns Sy 5 Ns % Pq ulong
+.It Sy zfs_per_txg_dirty_frees_percent Ns = Ns Sy 30 Ns % Pq ulong
 Control percentage of dirtied indirect blocks from frees allowed into one TXG.
 After this threshold is crossed, additional frees will wait until the next TXG.
 .Sy 0 No disables this throttle.

module/zfs/dmu.c

@@ -70,7 +70,7 @@ int zfs_nopwrite_enabled = 1;
  * will wait until the next TXG.
  * A value of zero will disable this throttle.
  */
-unsigned long zfs_per_txg_dirty_frees_percent = 5;
+static unsigned long zfs_per_txg_dirty_frees_percent = 30;
 
 /*
  * Enable/disable forcing txg sync when dirty in dmu_offset_next.