fs/netfs/read_collect: fix crash due to uninitialized `prev` variable

fs/netfs/read_collect: fix crash due to uninitialized `prev` variable

[ Just like the other two netfs patches I sent yesterday, this one
  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").  ]

When checking whether the edges of adjacent subrequests touch, the
`prev` variable is deferenced, but it might not have been initialized.
This causes crashes like this one:

 BUG: unable to handle page fault for address: 0000000181343843
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 8000001c66db0067 P4D 8000001c66db0067 PUD 0
 Oops: Oops: 0000 [#1] SMP PTI
 CPU: 1 UID: 33333 PID: 24424 Comm: php-cgi8.2 Kdump: loaded Not tainted 6.13.2-cm4all0-hp+ #427
 Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 11/23/2021
 RIP: 0010:netfs_consume_read_data.isra.0+0x5ef/0xb00
 Code: fe ff ff 48 8b 83 88 00 00 00 48 8b 4c 24 30 4c 8b 43 78 48 85 c0 48 8d 51 70 75 20 48 8b 73 30 48 39 d6 74 17 48 8b 7c 24 40 <48> 8b 4f 78 48 03 4f 68 48 39 4b 68 0f 84 ab 02 00 00 49 29 c0 48
 RSP: 0000:ffffc90037adbd00 EFLAGS: 00010283
 RAX: 0000000000000000 RBX: ffff88811bda0600 RCX: ffff888620e7b980
 RDX: ffff888620e7b9f0 RSI: ffff88811bda0428 RDI: 00000001813437cb
 RBP: 0000000000000000 R08: 0000000000004000 R09: 0000000000000000
 R10: ffffffff82e070c0 R11: 0000000007ffffff R12: 0000000000004000
 R13: ffff888620e7bb68 R14: 0000000000008000 R15: ffff888620e7bb68
 FS:  00007ff2e0e7ddc0(0000) GS:ffff88981f840000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000181343843 CR3: 0000001bc10ba006 CR4: 00000000001706f0
 Call Trace:
  <TASK>
  ? __die+0x1f/0x60
  ? page_fault_oops+0x15c/0x450
  ? search_extable+0x22/0x30
  ? netfs_consume_read_data.isra.0+0x5ef/0xb00
  ? search_module_extables+0xe/0x40
  ? exc_page_fault+0x5e/0x100
  ? asm_exc_page_fault+0x22/0x30
  ? netfs_consume_read_data.isra.0+0x5ef/0xb00
  ? intel_iommu_unmap_pages+0xaa/0x190
  ? __pfx_cachefiles_read_complete+0x10/0x10
  netfs_read_subreq_terminated+0x24f/0x390
  cachefiles_read_complete+0x48/0xf0
  iomap_dio_bio_end_io+0x125/0x160
  blk_update_request+0xea/0x3e0
  scsi_end_request+0x27/0x190
  scsi_io_completion+0x43/0x6c0
  blk_complete_reqs+0x40/0x50
  handle_softirqs+0xd1/0x280
  irq_exit_rcu+0x91/0xb0
  common_interrupt+0x3b/0xa0
  asm_common_interrupt+0x22/0x40
 RIP: 0033:0x55fe8470d2ab
 Code: 00 00 3c 7e 74 3b 3c b6 0f 84 dd 03 00 00 3c 1e 74 2f 83 c1 01 48 83 c2 38 48 83 c7 30 44 39 d1 74 3e 48 63 42 08 85 c0 79 a3 <49> 8b 46 48 8b 04 38 f6 c4 04 75 0b 0f b6 42 30 83 e0 0c 3c 04 75
 RSP: 002b:00007ffca5ef2720 EFLAGS: 00000216
 RAX: 0000000000000023 RBX: 0000000000000008 RCX: 000000000000001b
 RDX: 00007ff2e0cdb6f8 RSI: 0000000000000006 RDI: 0000000000000510
 RBP: 00007ffca5ef27a0 R08: 00007ffca5ef2720 R09: 0000000000000001
 R10: 000000000000001e R11: 00007ff2e0c10d08 R12: 0000000000000001
 R13: 0000000000000120 R14: 00007ff2e0cb1ed0 R15: 00000000000000b0
  </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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>