erofs: fix interlaced plain identification for encoded extents

In the Linux kernel, the following vulnerability has been resolved:

erofs: fix interlaced plain identification for encoded extents

Only plain data whose start position and on-disk physical length are

both aligned to the block size should be classified as interlaced

plain extents. Otherwise, it must be treated as shifted plain extents.

This issue was found by syzbot using a crafted compressed image

containing plain extents with unaligned physical lengths, which can

cause OOB read in z_erofs_transform_plain().