Xarray: distinguish large entries correctly in xas_split_alloc()

We don't support large entries which expand two more level xa_node in
split.  For case "xas->xa_shift + 2 * XA_CHUNK_SHIFT == order", we also
need two level of xa_node to expand.  Distinguish entry as large entry in
case "xas->xa_shift + 2 * XA_CHUNK_SHIFT == order".

As max order of folio in pagecache (MAX_PAGECACHE_ORDER) is <=
(XA_CHUNK_SHIFT * 2 - 1), this change is more likely a cleanup...

Link: https://lkml.kernel.org/r/20241213122523.12764-4-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Mattew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>