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 ce63cb6 ("erofs: support
unencoded inodes for fileio"), but was practically unreachable because
there was room for 256 folios in the `struct bio` - until commit
9f74ae8 ("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.

Fixes: ce63cb6 ("erofs: support unencoded inodes for fileio")
Fixes: 9f74ae8 ("erofs: shorten bvecs[] for file-backed mounts")
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Gao Xiang <xiang@kernel.org>
Tested-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20250428230933.3422273-1-max.kellermann@ionos.com
Signed-off-by: Gao Xiang <xiang@kernel.org>

Author: MaxKellermann

fs/erofs/fileio.c

@@ -150,10 +150,10 @@ static int erofs_fileio_scan_folio(struct erofs_fileio *io, struct folio *folio)
 				io->rq->bio.bi_iter.bi_sector = io->dev.m_pa >> 9;
 				attached = 0;
 			}
-			if (!attached++)
-				erofs_onlinefolio_split(folio);
 			if (!bio_add_folio(&io->rq->bio, folio, len, cur))
 				goto io_retry;
+			if (!attached++)
+				erofs_onlinefolio_split(folio);
 			io->dev.m_pa += len;
 		}
 		cur += len;