btrfs: fix wrong bvec_len in btrfs_csum_one_bio()

Fixed below WARNING.

WARNING: CPU: 0 PID: 511 at /home/mlin/linux/fs/btrfs/ordered-data.c:288
btrfs_add_ordered_sum+0x5e/0x8e()
Modules linked in:
CPU: 0 PID: 511 Comm: kworker/u2:4 Not tainted 3.18.0-00029-g30fd029 #68
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
01/01/2011
Workqueue: btrfs-worker btrfs_worker_helper
0000000000000009 ffff88001e357c58 ffffffff81488839 0000000000000002
0000000000000000 ffff88001e357c98 ffffffff8103b8a3 ffff88001e357c88
ffffffff8124ec83 ffff88001ddd4000 ffff88001d8e8000 ffff88001df248a0
Call Trace:
[<ffffffff81488839>] dump_stack+0x4f/0x7c
[<ffffffff8103b8a3>] warn_slowpath_common+0x81/0x9b
[<ffffffff8124ec83>] ? btrfs_add_ordered_sum+0x5e/0x8e
[<ffffffff8103b8d7>] warn_slowpath_null+0x1a/0x1c
[<ffffffff8124ec83>] btrfs_add_ordered_sum+0x5e/0x8e
[<ffffffff8122e895>] btrfs_csum_one_bio+0x16a/0x3de
[<ffffffff81232929>] ? btrfs_async_submit_limit+0x28/0x28
[<ffffffff8123bebb>] __btrfs_submit_bio_start+0x1d/0x27
[<ffffffff81230d37>] run_one_async_start+0x27/0x32
[<ffffffff81261932>] normal_work_helper+0xf9/0x2fd
[<ffffffff81261b48>] btrfs_worker_helper+0x12/0x14
[<ffffffff8104ded7>] process_one_work+0x1ca/0x376
[<ffffffff8104e319>] worker_thread+0x267/0x366
[<ffffffff8104e0b2>] ? process_scheduled_works+0x2f/0x2f
[<ffffffff81051e7c>] kthread+0xd2/0xda
[<ffffffff81480000>] ? br_multicast_set_hash_max+0x4d/0xcd
[<ffffffff81051daa>] ? kthread_freezable_should_stop+0x48/0x48
[<ffffffff8148da6c>] ret_from_fork+0x7c/0xb0
[<ffffffff81051daa>] ? kthread_freezable_should_stop+0x48/0x48

Signed-off-by: Ming Lin <minggr.net>
block: plugging

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
[dpark: fix compile errors]
md: get rid of bio_list from struct task
blk-mq: remove also remaining calls to blk_attempt_plug_merge()
block: fix panic in generic_make_request()

bio should be added to splits.
Fixed below panic.

[    1.112557] BUG: unable to handle kernel NULL pointer dereference at           (null)
[    1.112563] IP: [<ffffffff812bc2d3>] generic_make_request+0x63/0x9d
[    1.112564] PGD 0
[    1.112566] Oops: 0002 [#1] PREEMPT SMP
[    1.112568] Modules linked in:
[    1.112570] CPU: 3 PID: 530 Comm: kworker/u8:2 Not tainted 3.18.0-00033-ge76e0b3 #41
[    1.112571] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[    1.112575] Workqueue: events_unbound async_run_entry_fn
[    1.112576] task: ffff88001d8eae90 ti: ffff88001dad0000 task.ti: ffff88001dad0000
[    1.112579] RIP: 0010:[<ffffffff812bc2d3>]  [<ffffffff812bc2d3>] generic_make_request+0x63/0x9d
[    1.112579] RSP: 0000:ffff88001dad3798  EFLAGS: 00010246
[    1.112580] RAX: 0000000000000000 RBX: ffff88001d8eae90 RCX: 0000000000000008
[    1.112581] RDX: ffff88001dad3798 RSI: 0000000000000000 RDI: ffff88001dad3798
[    1.112582] RBP: ffff88001dad37d8 R08: ffff88001e2908b8 R09: 000000000000000d
[    1.112583] R10: 000000000000000d R11: 000000000000000c R12: ffff88001dad3798
[    1.112583] R13: 0000000000000000 R14: 0000000000000000 R15: ffffea000078cb80
[    1.112585] FS:  0000000000000000(0000) GS:ffff88001fd80000(0000) knlGS:0000000000000000
[    1.112586] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[    1.112588] CR2: 0000000000000000 CR3: 000000000180e000 CR4: 00000000000006e0
[    1.112593] Stack:
[    1.112595]  0000000000000000 0000000000000000 ffff88001dad37a8 ffff88001dad37a8
[    1.112597]  ffff88001dad37b8 ffff88001dad37b8 ffff88001e290840 0000000000000008
[    1.112599]  ffff88001dad3838 ffffffff812bc404 ffff88001dad3808 0000000481059f5e
[    1.112599] Call Trace:
[    1.112603]  [<ffffffff812bc404>] submit_bio+0xf7/0x136
[    1.112607]  [<ffffffff8114a1b9>] _submit_bh+0x104/0x12e
[    1.112610]  [<ffffffff8114a1f3>] submit_bh+0x10/0x12
[    1.112612]  [<ffffffff8114a4be>] block_read_full_page+0x24a/0x266
[    1.112614]  [<ffffffff8114be05>] ? I_BDEV+0xd/0xd
[    1.112618]  [<ffffffff812e8bd2>] ? debug_smp_processor_id+0x17/0x19
[    1.112621]  [<ffffffff810dd817>] ? __lru_cache_add+0x72/0x88
[    1.112623]  [<ffffffff8114c4e8>] ? blkdev_readpages+0x1f/0x1f
[    1.112625]  [<ffffffff8114c500>] blkdev_readpage+0x18/0x1a
[    1.112628]  [<ffffffff810d2f1f>] do_read_cache_page+0x97/0x16f
[    1.112630]  [<ffffffff810d3013>] read_cache_page+0x1c/0x1e
[    1.112633]  [<ffffffff812c98cf>] read_dev_sector+0x2d/0x87
[    1.112636]  [<ffffffff812cc283>] read_lba+0xb1/0x105
[    1.112638]  [<ffffffff812cc6e4>] ? find_valid_gpt+0x80/0x4d4
[    1.112640]  [<ffffffff812cc705>] find_valid_gpt+0xa1/0x4d4
[    1.112642]  [<ffffffff812dd430>] ? string.isra.6+0x3d/0xa2
[    1.112645]  [<ffffffff812ccbb4>] efi_partition+0x7c/0x364
[    1.112647]  [<ffffffff812de3b4>] ? vsnprintf+0x82/0x3a7
[    1.112649]  [<ffffffff812de77f>] ? snprintf+0x34/0x36
[    1.112651]  [<ffffffff812ca773>] check_partition+0x11e/0x1c3
[    1.112653]  [<ffffffff812c9e53>] rescan_partitions+0x98/0x271
[    1.112657]  [<ffffffff814818c8>] ? _raw_spin_lock+0x18/0x37
[    1.112659]  [<ffffffff8114d32b>] __blkdev_get+0x1c7/0x3f6
[    1.112660]  [<ffffffff814818c8>] ? _raw_spin_lock+0x18/0x37
[    1.112663]  [<ffffffff8114d5b8>] blkdev_get+0x5e/0x2a9
[    1.112666]  [<ffffffff8113513b>] ? unlock_new_inode+0x5c/0x61
[    1.112668]  [<ffffffff8114c3cb>] ? bdget+0x120/0x12f
[    1.112670]  [<ffffffff81330a9e>] ? put_device+0x17/0x19
[    1.112672]  [<ffffffff812c7e17>] add_disk+0x286/0x3f9
[    1.112676]  [<ffffffff8136ab9f>] sd_probe_async+0x106/0x19d
[    1.112678]  [<ffffffff81053a9c>] async_run_entry_fn+0x71/0x138
[    1.112681]  [<ffffffff8104d603>] process_one_work+0x1c7/0x36f
[    1.112683]  [<ffffffff8104e090>] worker_thread+0x28b/0x37f
[    1.112685]  [<ffffffff8104de05>] ? rescuer_thread+0x226/0x226
[    1.112687]  [<ffffffff810519a8>] kthread+0xdb/0xe3
[    1.112689]  [<ffffffff81480000>] ? __ww_mutex_lock_slowpath+0xcc/0x1c3
[    1.112691]  [<ffffffff810518cd>] ? kthread_create_on_node+0x17f/0x17f
[    1.112693]  [<ffffffff8148226c>] ret_from_fork+0x7c/0xb0
[    1.112695]  [<ffffffff810518cd>] ? kthread_create_on_node+0x17f/0x17f

Signed-off-by: Ming Lin <mlin@minggr.net>