xfs_scrub: track repair items by principal, not by individual repairs

xfs_scrub: track repair items by principal, not by individual repairs

Create a new structure to track scrub and repair state by principal
filesystem object (e.g. ag number or inode number/generation) so that we
can more easily examine and ensure that we satisfy repair order
dependencies.  This transposition will eventually enable bulk scrub
operations and will also save a lot of memory if a given object needs a
lot of work.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>