fs/erofs/fileio: call erofs_onlinefolio_split() after bio_add_folio()

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

fs/erofs/fileio: call erofs_onlinefolio_split() after bio_add_folio()

If bio_add_folio() fails (because it is full),

erofs_fileio_scan_folio() needs to submit the I/O request via

erofs_fileio_rq_submit() and allocate a new I/O request with an empty

`struct bio`. Then it retries the bio_add_folio() call.

However, at this point, erofs_onlinefolio_split() has already been

called which increments `folio->private`; the retry will call

erofs_onlinefolio_split() again, but there will never be a matching

erofs_onlinefolio_end() call. This leaves the folio locked forever

and all waiters will be stuck in folio_wait_bit_common().

This bug has been added by commit ce63cb62d794 ("erofs: support

unencoded inodes for fileio"), but was practically unreachable because

there was room for 256 folios in the `struct bio` - until commit

9f74ae8c9ac9 ("erofs: shorten bvecs[] for file-backed mounts") which

reduced the array capacity to 16 folios.

It was now trivial to trigger the bug by manually invoking readahead

from userspace, e.g.:

posix_fadvise(fd, 0, st.st_size, POSIX_FADV_WILLNEED);

This should be fixed by invoking erofs_onlinefolio_split() only after

bio_add_folio() has succeeded. This is safe: asynchronous completions

invoking erofs_onlinefolio_end() will not unlock the folio because

erofs_fileio_scan_folio() is still holding a reference to be released

by erofs_onlinefolio_end() at the end.