Restrict raidz faulted vdev count

Specifically, a child in a replacing vdev won't count when assessing
the dtl during a vdev_fault()

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #16569