Fix horrible, embarrassing breakage which caused occasional panics and data corruption.  It's a wonder it worked at all.
Fix horrible, embarrassing breakage which caused occasional panics and
data corruption.  It's a wonder it worked at all.

Led-on-the-right-path-by:  dillon

revive_block: Add treatment for RAID-4.

Add function parityrebuild, called by revive_block and parityops.

Approved-by:  jkh

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