erofs: fix order >= MAX_ORDER warning due to crafted negative i_size

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

erofs: fix order >= MAX_ORDER warning due to crafted negative i_size

As syzbot reported [1], the root cause is that i_size field is a signed type, and negative i_size is also less than EROFS_BLKSIZ. As a consequence, it's handled as fast symlink unexpectedly.

Let's fall back to the generic path to deal with such unusual i_size.

[1] https://lore.kernel.org/r/000000000000ac8efa05e7feaa1f@google.com