zfs: fix RAIDZ2/3 not healing parity with 2+ bad disks
From openzfs-master 62d4287f2 commit message:
When scrubbing, (non-sequential) resilvering, or correcting a checksum error using RAIDZ parity, ZFS should heal any incorrect RAIDZ parity by overwriting it. For example, if P disks are silently corrupted (P being the number of failures tolerated; e.g. RAIDZ2 has P=2), `zpool scrub` should detect and heal all the bad state on these disks, including parity. This way if there is a subsequent failure we are fully protected. With RAIDZ2 or RAIDZ3, a block can have silent damage to a parity sector, and also damage (silent or known) to a data sector. In this case the parity should be healed but it is not.
Cherry-picked from openzfs 62d4287f279a0d184f8f332475f27af58b7aa87e
Patch Author: Matthew Ahrens <matthew.ahrens@delphix.com>
MFC after: 3 days
Reviewed by: delphij
Differential Revision: https://reviews.freebsd.org/D28681