block: check queue's limits.discard_granularity in __blkdev_issue_discard()

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...