xfs: fix bugs in parent pointer checking [v24.5]

Jan Kara pointed out that the VFS doesn't take i_rwsem of a child
subdirectory that is being moved from one parent to another.  Upon
deeper analysis, I realized that this was the source of a very hard to
trigger false corruption report in the parent pointer checking code.

Now that we've refactored how directory walks work in scrub, we can also
get rid of all the unnecessary and broken locking to make parent pointer
scrubbing work properly.

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