xfs_scrub: schedule and manage optimizations/repairs to the filesystem

xfs_scrub: schedule and manage optimizations/repairs to the filesystem

Teach xfs_scrub to remember scrub requests that failed (or indicated
that optimization is a possibility) as action items.  Depending on the
circumstances, certain items are acted upon immediately (e.g. metadata
that needs to be healthy in order to finish the scan, or files that are
already open) or deferred until later.  Expand the repair phase to
deal with the deferred actions.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>