Improve scrub maxinflight_bytes math.

Previously, ZFS scaled maxinflight_bytes based on total number of
disks in the pool.  A 3-wide mirror was receiving a queue depth of 3
disks, which it should not, since it reads from all the disks inside.
For wide raidz the situation was slightly better, but still a 3-wide
raidz1 received a depth of 3 disks instead of 2.

The new code counts only unique data disks, i.e. 1 disk for mirrors
and non-parity disks for raidz/draid.  For draid the math is still
imperfect, since vdev_get_nparity() returns number of parity disks
per group, not per vdev, but still some better than it was.

This should slightly reduce scrub influence on payload for some pool
topologies by avoiding excessive queuing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored-By:	iXsystems, Inc.
Closing #12046

man/man5/zfs-module-parameters.5

@@ -3326,7 +3326,7 @@ Default value: \fB0\fR.
 Maximum amount of data that can be concurrently issued at once for scrubs and
 resilvers per leaf device, given in bytes.
 .sp
-Default value: \fB41943040\fR.
+Default value: \fB4194304\fR.
 .RE
 
 .sp