Xarray: move forward index correctly in xas_pause()

After xas_load(), xas->index could point to mid of found multi-index entry
and xas->index's bits under node->shift maybe non-zero.  The afterward
xas_pause() will move forward xas->index with xa->node->shift with bits
under node->shift un-masked and thus skip some index unexpectedly.

Consider following case:
Assume XA_CHUNK_SHIFT is 4.
xa_store_range(xa, 16, 31, ...)
xa_store(xa, 32, ...)
XA_STATE(xas, xa, 17);
xas_for_each(&xas,...)
xas_load(&xas)
/* xas->index = 17, xas->xa_offset = 1, xas->xa_node->xa_shift = 4 */
xas_pause()
/* xas->index = 33, xas->xa_offset = 2, xas->xa_node->xa_shift = 4 */
As we can see, index of 32 is skipped unexpectedly.

Fix this by mask bit under node->xa_shift when move forward index in
xas_pause().

For now, this will not cause serious problems.  Only minor problem like
cachestat return less number of page status could happen.

Link: https://lkml.kernel.org/r/20241213122523.12764-3-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>