[BUG] bcache: BUG_ON in bch_prio_write()

[Farhad Alemi]

Hello Coly Li, Kent Overstreet,

While fuzzing Linux 7.1-rc5 with syzkaller, as part of research at ASU's
SEFCOM lab, we hit the crash below. Crash reports can be found here:

  https://github.com/farhad-alemi/public_bug_reports/tree/main/115-bcache-bug_on-bch_prio_write/

  ------------[ cut here ]------------
  kernel BUG at drivers/md/bcache/super.c:660!
  Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
  CPU: 0 UID: 0 PID: 16493 Comm: bcache_allocato Not tainted 7.1.0-rc5
#1 PREEMPT(full)
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
1.16.3-debian-1.16.3-2 04/01/2014
  RIP: 0010:bch_prio_write+0x10a6/0x10c0 drivers/md/bcache/super.c:660
  Code: f9 e9 b1 f5 ff ff 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 98 f3
ff ff 4c 89 f7 e8 e5 62 e8 f9 e9 8b f3 ff ff e8 0b f9 80 f9 90 <0f> 0b
e8 03 f9 80 f9 e8 ee fe f4 f8 e9 0e f2 ff ff 66 0f 1f 84 00
  RSP: 0000:ffffc90006657a40 EFLAGS: 00010293
  RAX: ffffffff883dceb5 RBX: ffffffffffffffff RCX: ffff88801d0c9b00

Our reproducer.c is available upon request.

Happy to test a patch if that would help.

Regards,