mm: shmem: fix incorrect index alignment for within_size policy

commit d0e6983a6d1719738cf8d13982a68094f0a1872a upstream.

With enabling the shmem per-size within_size policy, using an incorrect
'order' size to round_up() the index can lead to incorrect i_size checks,
resulting in an inappropriate large orders being returned.

Changing to use '1 << order' to round_up() the index to fix this issue.
Additionally, adding an 'aligned_index' variable to avoid affecting the
index checks.

Link: https://lkml.kernel.org/r/77d8ef76a7d3d646e9225e9af88a76549a68aab1.1734593154.git.baolin.wang@linux.alibaba.com


Fixes: e7a2ab7b ("mm: shmem: add mTHP support for anonymous shmem")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>