xfs: scrub/repair update health tracking

This series enhances the online scrub and repair code to report their
findings to the health tracking subsystem.  For now, only scrub gets to
record corruption problems, though in principle a metadata read
encountering corruption could also set a sick flag.

Online repair will clear the appropriate sick flags when metadata passes
its inspection after a repair attempt.

The first three patches rework the scrub context management code to
remove some clunkiness when we decide that we have to start the scrub
over.

Patch #4 actually teaches scrub to update the health subsystem.

Patch #5 teaches scrub to skip cross-referencing with known corrupt metadata.