xfs: online repair of file link counts

Now that we've created the infrastructure to perform live scans of every
file in the filesystem and the necessary hook infrastructure to observe
live updates, use it to scan directories to compute the correct link
counts for files in the filesystem, and reset those link counts.

This patchset creates a tailored readdir implementation for scrub
because the regular version has to cycle ILOCKs to copy information to
userspace.  We can't cycle the ILOCK during the nlink scan and we don't
need all the other VFS support code (maintaining a readdir cursor and
translating XFS structures to VFS structures and back) so it was easier
to duplicate the code.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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