If create a loop device with a backing NVMe SSD, current loop device
driver doesn't correctly set its  queue's limits.discard_granularity and
leaves it as 0. If a discard request at LBA 0 on this loop device, in
__blkdev_issue_discard() the calculated req_sects will be 0, and a zero
length discard request will trigger a BUG() panic in generic block layer
code at block/blk-mq.c:563.

[  955.565006][   C39] ------------[ cut here ]------------
[  955.559660][   C39] invalid opcode: 0000 [#1] SMP NOPTI
[  955.622171][   C39] CPU: 39 PID: 248 Comm: ksoftirqd/39 Tainted: G            E     5.8.0-default+ #40
[  955.622171][   C39] Hardware name: Lenovo ThinkSystem SR650 -[7X05CTO1WW]-/-[7X05CTO1WW]-, BIOS -[IVE160M-2.70]- 07/17/2020
[  955.622175][   C39] RIP: 0010:blk_mq_end_request+0x107/0x110
[  955.622177][   C39] Code: 48 8b 03 e9 59 ff ff ff 48 89 df 5b 5d 41 5c e9 9f ed ff ff 48 8b 35 98 3c f4 00 48 83 c7 10 48 83 c6 19 e8 cb 56 c9 ff eb cb <0f> 0b 0f 1f 80 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 41 56 41 54
[  955.622179][   C39] RSP: 0018:ffffb1288701fe28 EFLAGS: 00010202
[  955.749277][   C39] RAX: 0000000000000001 RBX: ffff956fffba5080 RCX: 0000000000004003
[  955.749278][   C39] RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000000
[  955.749279][   C39] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[  955.749279][   C39] R10: ffffb1288701fd28 R11: 0000000000000001 R12: ffffffffa8e05160
[  955.749280][   C39] R13: 0000000000000004 R14: 0000000000000004 R15: ffffffffa7ad3a1e
[  955.749281][   C39] FS:  0000000000000000(0000) GS:ffff95bfbda00000(0000) knlGS:0000000000000000
[  955.749282][   C39] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  955.749282][   C39] CR2: 00007f6f0ef766a8 CR3: 0000005a37012002 CR4: 00000000007606e0
[  955.749283][   C39] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  955.749284][   C39] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  955.749284][   C39] PKRU: 55555554
[  955.749285][   C39] Call Trace:
[  955.749290][   C39]  blk_done_softirq+0x99/0xc0
[  957.550669][   C39]  __do_softirq+0xd3/0x45f
[  957.550677][   C39]  ? smpboot_thread_fn+0x2f/0x1e0
[  957.550679][   C39]  ? smpboot_thread_fn+0x74/0x1e0
[  957.550680][   C39]  ? smpboot_thread_fn+0x14e/0x1e0
[  957.550684][   C39]  run_ksoftirqd+0x30/0x60
[  957.550687][   C39]  smpboot_thread_fn+0x149/0x1e0
[  957.886225][   C39]  ? sort_range+0x20/0x20
[  957.886226][   C39]  kthread+0x137/0x160
[  957.886228][   C39]  ? kthread_park+0x90/0x90
[  957.886231][   C39]  ret_from_fork+0x22/0x30
[  959.117120][   C39] ---[ end trace 3dacdac97e2ed164 ]---

This is the procedure to reproduce the panic,
  # modprobe scsi_debug delay=0 dev_size_mb=2048 max_queue=1
  # losetup -f /dev/nvme0n1 --direct-io=on
  # blkdiscard /dev/loop0 -o 0 -l 0x200

This patch fixes the issue by checking q->limits.discard_granularity in
__blkdev_issue_discard() before composing the discard bio. If the value
is 0, then prints a warning oops information and returns -EOPNOTSUPP to
the caller to indicate that this buggy device driver doesn't support
discard request.

Fixes: 9b15d109 ("block: improve discard bio alignment in __blkdev_issue_discard()")
Fixes: c52abf56 ("loop: Better discard support for block devices")
Reported-and-suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Enzo Matsumiya <ematsumiya@suse.com>
Cc: Evan Green <evgreen@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Xiao Ni <xni@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>