xfs: retain ILOCK during directory updates [v13.0 37/63]

This series changes the directory update code to retain the ILOCK on all
files involved in a rename until the end of the operation.  The upcoming
parent pointers patchset applies parent pointers in a separate chained
update from the actual directory update, which is why it is now
necessary to keep the ILOCK instead of dropping it after the first
transaction in the chain.

As a side effect, we no longer need to hold the IOLOCK during an rmapbt
scan of inodes to serialize the scan with ongoing directory updates.

This has been running on the djcloud for months with no problems.  Enjoy!

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