lib/xarray: introduce a new helper xas_get_order

lib/xarray: introduce a new helper xas_get_order

It can be used after xas_load to check the order of loaded entries.
Compared to xa_get_order, it saves an XA_STATE and avoid a rewalk.

Added new test for xas_get_order, to make the test work, we have to export
xas_get_order with EXPORT_SYMBOL_GPL.

Also fix a sparse warning by checking the slot value with xa_entry instead
of accessing it directly, as suggested by Matthew Wilcox.

[kasong@tencent.com: simplify comment, sparse warning fix, per Matthew Wilcox]
Link: https://lkml.kernel.org/r/20240416071722.45997-4-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20240415171857.19244-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>