fs/nfs/read: fix double-unlock bug in nfs_return_empty_folio()

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

fs/nfs/read: fix double-unlock bug in nfs_return_empty_folio()

Sometimes, when a file was read while it was being truncated by

another NFS client, the kernel could deadlock because folio_unlock()

was called twice, and the second call would XOR back the `PG_locked`

flag.

Most of the time (depending on the timing of the truncation), nobody

notices the problem because folio_unlock() gets called three times,

which flips `PG_locked` back off:

1. vfs_read, nfs_read_folio, ... nfs_read_add_folio,

nfs_return_empty_folio

2. vfs_read, nfs_read_folio, ... netfs_read_collection,

netfs_unlock_abandoned_read_pages

3. vfs_read, ... nfs_do_read_folio, nfs_read_add_folio,

nfs_return_empty_folio

The problem is that nfs_read_add_folio() is not supposed to unlock the

folio if fscache is enabled, and a nfs_netfs_folio_unlock() check is

missing in nfs_return_empty_folio().

Rarely this leads to a warning in netfs_read_collection():

------------[ cut here ]------------

R=0000031c: folio 10 is not locked

WARNING: CPU: 0 PID: 29 at fs/netfs/read_collect.c:133 netfs_read_collection+0x7c0/0xf00

[...]

Workqueue: events_unbound netfs_read_collection_worker

RIP: 0010:netfs_read_collection+0x7c0/0xf00

[...]

Call Trace:

<TASK>

netfs_read_collection_worker+0x67/0x80

process_one_work+0x12e/0x2c0

worker_thread+0x295/0x3a0

Most of the time, however, processes just get stuck forever in

folio_wait_bit_common(), waiting for `PG_locked` to disappear, which

never happens because nobody is really holding the folio lock.