MFV r323789: 8473 scrub does not detect errors on active spares

MFV r323789:  8473 scrub does not detect errors on active spares

illumos/illumos-gate@554675eee75dd2d7398d960aa5c81083ceb8505a
https://github.com/illumos/illumos-gate/commit/554675eee75dd2d7398d960aa5c81083ceb8505a

https://www.illumos.org/issues/8473
Scrubbing is supposed to detect and repair all errors in the pool. However,
it wrongly ignores active spare devices. The problem can easily be
reproduced in OpenZFS at git rev 0ef125d with these commands:

truncate -s 64m /tmp/a /tmp/b /tmp/c
sudo zpool create testpool mirror /tmp/a /tmp/b spare /tmp/c
sudo zpool replace testpool /tmp/a /tmp/c
/bin/dd if=/dev/zero bs=1024k count=63 oseek=1 conv=notrunc of=/tmp/c
sync
sudo zpool scrub testpool
zpool status testpool # Will show 0 errors, which is wrong
sudo zpool offline testpool /tmp/a
sudo zpool scrub testpool
zpool status testpool # Will show errors on /tmp/c,
		      # which should've already been fixed

FreeBSD head is partially affected: the first scrub will detect some errors, but the second scrub will detect more.

Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

MFC after:	1 week
Sponsored by:	Spectra Logic Corp