test_xarray: add tests for advanced multi-index use

Patch series "test_xarray: advanced API multi-index tests", v2.

This is a respin of the test_xarray multi-index tests [0] which use and
demonstrate the advanced API which is used by the page cache.  This should
let folks more easily follow how we use multi-index to support for example
a min order later in the page cache.  It also lets us grow the selftests
to mimic more of what we do in the page cache.


This patch (of 2):

The multi index selftests are great but they don't replicate how we deal
with the page cache exactly, which makes it a bit hard to follow as the
page cache uses the advanced API.

Add tests which use the advanced API, mimicking what we do in the page
cache, while at it, extend the example to do what is needed for min order
support.

[mcgrof@kernel.org: fix soft lockup for advanced-api tests]
  Link: https://lkml.kernel.org/r/20240216194329.840555-1-mcgrof@kernel.org
[akpm@linux-foundation.org: s/i/loops/, make non-static]
[akpm@linux-foundation.org: restore static storage for loop counter]
Link: https://lkml.kernel.org/r/20240131225125.1370598-1-mcgrof@kernel.org
Link: https://lkml.kernel.org/r/20240131225125.1370598-2-mcgrof@kernel.org


Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Tested-by: Daniel Gomez <da.gomez@samsung.com>
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>