Backport request: commit 0dc54bd4d6e0 ("fscache_cookie_enabled: check cookie is valid before accessing it")

[Jeffrey E Altman]

Please backport commit 0dc54bd4d6e03be1f0b678c4297170b79f1a44ab
("fscache_cookie_enabled: check cookie is valid before accessing it") to
the 5.13, 5.14, and 5.15 kernel series.

Commit 0dc54bd4d6e03be1f0b678c4297170b79f1a44ab fixes a bug introduced
by 3003bbd0697b659944237f3459489cb596ba196c ("afs: Use the
netfs_write_begin() helper") that results in a NULL pointer dereference
observed in Fedora 35 when accessing afs volumes from Kubernetes.

[ 3627.403829] BUG: kernel NULL pointer dereference, address:
0000000000000068
[ 3627.411649] RIP: 0010:afs_is_cache_enabled+0xc/0x30 [kafs]
[ 3627.419900] Call Trace:
[ 3627.420432]  <TASK>
[ 3627.420957]  netfs_write_begin+0x1ff/0x810 [netfs]
[ 3627.421498]  ? lock_timer_base+0x61/0x80
[ 3627.422124]  afs_write_begin+0x58/0x240 [kafs]
[ 3627.422738]  generic_perform_write+0xae/0x1d0
[ 3627.423325]  ? file_update_time+0xd2/0x120
[ 3627.423806]  __generic_file_write_iter+0x101/0x1d0
[ 3627.424275]  generic_file_write_iter+0x5d/0xb0
[ 3627.424741]  afs_file_write+0x73/0xa0 [kafs]
[ 3627.425270]  new_sync_write+0x10b/0x180
[ 3627.425708]  vfs_write+0x1ce/0x260
[ 3627.426160]  ksys_write+0x4f/0xc0
[ 3627.426606]  do_syscall_64+0x3b/0x90
[ 3627.427086]  entry_SYSCALL_64_after_hwframe+0x44/0xae

The defect was introduced in v5.13-rc1 and fixed in v5.16-rc1.

Thank you and Happy New Year.

Jeffrey Altman

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