fs/netfs/read_pgpriv2: skip folio queues without `marks3`

fs/netfs/read_pgpriv2: skip folio queues without `marks3`

[ Note this patch doesn't apply to v6.14 as it was obsoleted by commit
  e2d46f2ec332 ("netfs: Change the read result collector to only use one
  work item"). ]

At the beginning of the function, folio queues with marks3==0 are
skipped, but after that, the `marks3` field is ignored.  If one such
queue is found, `slot` is set to 64 (because `__ffs(0)==64`), leading
to a buffer overflow in the folioq_folio() call.  The resulting crash
may look like this:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0000 [#1] SMP PTI
 CPU: 11 UID: 0 PID: 2909 Comm: kworker/u262:1 Not tainted 6.13.1-cm4all2-vm #415
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
 Workqueue: events_unbound netfs_read_termination_worker
 RIP: 0010:netfs_pgpriv2_write_to_the_cache+0x15a/0x3f0
 Code: 48 85 c0 48 89 44 24 08 0f 84 24 01 00 00 48 8b 80 40 01 00 00 48 8b 7c 24 08 f3 48 0f bc c0 89 44 24 18 89 c0 48 8b 74 c7 08 <48> 8b 06 48 c7 04 24 00 10 00 00 a8 40 74 10 0f b6 4e 40 b8 00 10
 RSP: 0018:ffffbbc440effe18 EFLAGS: 00010203
 RAX: 0000000000000040 RBX: ffff96f8fc034000 RCX: 0000000000000000
 RDX: 0000000000000040 RSI: 0000000000000000 RDI: ffff96f8fc036400
 RBP: 0000000000001000 R08: ffff96f9132bb400 R09: 0000000000001000
 R10: ffff96f8c1263c80 R11: 0000000000000003 R12: 0000000000001000
 R13: ffff96f8fb75ade8 R14: fffffaaf5ca90000 R15: ffff96f8fb75ad00
 FS:  0000000000000000(0000) GS:ffff9703cf0c0000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 000000010c9ca003 CR4: 00000000001706b0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  <TASK>
  ? __die+0x1f/0x60
  ? page_fault_oops+0x158/0x450
  ? search_extable+0x22/0x30
  ? netfs_pgpriv2_write_to_the_cache+0x15a/0x3f0
  ? search_module_extables+0xe/0x40
  ? exc_page_fault+0x62/0x120
  ? asm_exc_page_fault+0x22/0x30
  ? netfs_pgpriv2_write_to_the_cache+0x15a/0x3f0
  ? netfs_pgpriv2_write_to_the_cache+0xf6/0x3f0
  netfs_read_termination_worker+0x1f/0x60
  process_one_work+0x138/0x2d0
  worker_thread+0x2a5/0x3b0
  ? __pfx_worker_thread+0x10/0x10
  kthread+0xba/0xe0
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x30/0x50
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1a/0x30
  </TASK>

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>