xarray: add a BUG_ON() to ensure caller is not sibling

Suppose xas is pointing somewhere near the end of the multi-entry batch.
Then it may happen that the computed slot already falls beyond the batch,
thus breaking the loop due to !xa_is_sibling(), and computing the wrong
order.

For example, suppose we have a shift-6 node having an order-9 entry => 8 -
1 = 7 siblings, so assume the slots are at offset 0 till 7 in this node.
If xas->xa_offset is 6, then the code will compute order as 1 +
xas->xa_node->shift = 7. Therefore, the order computation must start from
the beginning of the multi-slot entries, that is, the non-sibling entry.

Thus ensure that the caller is aware of this by triggering a BUG when the
entry is a sibling entry. Note that this BUG_ON() is only active while
running selftests, so there is no overhead in a running kernel.

Link: https://lkml.kernel.org/r/20250604041533.91198-1-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>