xfs: implement live updates for directory repairs

xfs: implement live updates for directory repairs

While we're scanning the filesystem for parent pointers that we can turn
into dirents, we cannot hold the IOLOCK or ILOCK of the directory being
repaired.  Therefore, we need to set up a dirent hook so that we can
keep the temporary directory up to date with the rest of the filesystem.
Hence we add the ability to *remove* entries from the temporary dir.

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