scsi: core: Fix warning in scsi_alloc_sgtables()

scsi: core: Fix warning in scsi_alloc_sgtables()

As explained in SG_IO howto[1]:

"If iovec_count is non-zero then 'dxfer_len' should be equal to the sum of
iov_len lengths. If not, the minimum of the two is the transfer length."

When iovec_count is non-zero and dxfer_len is zero, the sg_io() just
genarated a null bio, and finally caused a warning below. To fix it, skip
generating a bio for this request if dxfer_len is zero.

[1] https://tldp.org/HOWTO/SCSI-Generic-HOWTO/x198.html

WARNING: CPU: 2 PID: 3643 at drivers/scsi/scsi_lib.c:1032 scsi_alloc_sgtables+0xc7d/0xf70 drivers/scsi/scsi_lib.c:1032
Modules linked in:

CPU: 2 PID: 3643 Comm: syz-executor397 Not tainted
5.17.0-rc3-syzkaller-00316-gb81b1829e7e3 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-204/01/2014
RIP: 0010:scsi_alloc_sgtables+0xc7d/0xf70 drivers/scsi/scsi_lib.c:1032
Code: e7 fc 31 ff 44 89 f6 e8 c1 4e e7 fc 45 85 f6 0f 84 1a f5 ff ff e8
93 4c e7 fc 83 c5 01 0f b7 ed e9 0f f5 ff ff e8 83 4c e7 fc <0f> 0b 41
   bc 0a 00 00 00 e9 2b fb ff ff 41 bc 09 00 00 00 e9 20 fb
RSP: 0018:ffffc90000d07558 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff88801bfc96a0 RCX: 0000000000000000
RDX: ffff88801c876000 RSI: ffffffff849060bd RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff849055b9 R11: 0000000000000000 R12: ffff888012b8c000
R13: ffff88801bfc9580 R14: 0000000000000000 R15: ffff88801432c000
FS:  00007effdec8e700(0000) GS:ffff88802cc00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007effdec6d718 CR3: 00000000206d6000 CR4: 0000000000150ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 scsi_setup_scsi_cmnd drivers/scsi/scsi_lib.c:1219 [inline]
 scsi_prepare_cmd drivers/scsi/scsi_lib.c:1614 [inline]
 scsi_queue_rq+0x283e/0x3630 drivers/scsi/scsi_lib.c:1730
 blk_mq_dispatch_rq_list+0x6ea/0x22e0 block/blk-mq.c:1851
 __blk_mq_sched_dispatch_requests+0x20b/0x410 block/blk-mq-sched.c:299
 blk_mq_sched_dispatch_requests+0xfb/0x180 block/blk-mq-sched.c:332
 __blk_mq_run_hw_queue+0xf9/0x350 block/blk-mq.c:1968
 __blk_mq_delay_run_hw_queue+0x5b6/0x6c0 block/blk-mq.c:2045
 blk_mq_run_hw_queue+0x30f/0x480 block/blk-mq.c:2096
 blk_mq_sched_insert_request+0x340/0x440 block/blk-mq-sched.c:451
 blk_execute_rq+0xcc/0x340 block/blk-mq.c:1231
 sg_io+0x67c/0x1210 drivers/scsi/scsi_ioctl.c:485
 scsi_ioctl_sg_io drivers/scsi/scsi_ioctl.c:866 [inline]
 scsi_ioctl+0xa66/0x1560 drivers/scsi/scsi_ioctl.c:921
 sd_ioctl+0x199/0x2a0 drivers/scsi/sd.c:1576
 blkdev_ioctl+0x37a/0x800 block/ioctl.c:588
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:874 [inline]
 __se_sys_ioctl fs/ioctl.c:860 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7effdecdc5d9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 81 14 00 00 90 48 89 f8 48 89
f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01
f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007effdec8e2f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007effded664c0 RCX: 00007effdecdc5d9
RDX: 0000000020002300 RSI: 0000000000002285 RDI: 0000000000000004
RBP: 00007effded34034 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 00007effded34054 R14: 2f30656c69662f2e R15: 00007effded664c8

Link: https://lore.kernel.org/r/20220720025120.3226770-1-yanaijie@huawei.com
Fixes: 09a5fc218fb8 ("block: fix SG_IO vector request data length handling")
Reported-by: syzbot+d44b35ecfb807e5af0b5@syzkaller.appspotmail.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>