XArray: add cmpxchg order test

XArray multi-index entries do not keep track of the order stored once the
entry is being marked as used with cmpxchg (conditionally replaced with
NULL).  Add a test to check the order is actually lost.  The test also
verifies the order and entries for all the tied indexes before and after
the NULL replacement with xa_cmpxchg.

Add another entry at 1 << order that keeps the node around and the order
information for the NULL-entry after xa_cmpxchg.

Link: https://lkml.kernel.org/r/20240131225125.1370598-3-mcgrof@kernel.org


Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>