xfs: fix corruptions in the directory tree

Repair corruptions in the directory tree itself. Cycles are broken by
removing an incoming parent->child link. Multiply-owned directories are
fixed by pruning the extra parent -> child links Disconnected subtrees
are reconnected to the lost and found.

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

xfs: invalidate dirloop scrub path data when concurrent updates happen

Add a dirent update hook so that we can detect directory tree updates
that affect any of the paths found by this scrubber and force it to
rescan.

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

xfs: teach online scrub to find directory tree structure problems

Create a new scrubber that detects corruptions within the directory tree
structure itself. It can detect directories with multiple parents;
loops within the directory tree; and directory loops not accessible from
the root.

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