md: use a mutex to protect a global list

We saw a list corruption in the list all_detected_devices:

 WARNING: CPU: 16 PID: 226 at lib/list_debug.c:29 __list_add+0x3c/0xa9()
 list_add corruption. next->prev should be prev (ffff880859d58320), but was ffff880859ce74c0. (next=ffffffff81abfdb0).
 Modules linked in: ahci libahci libata sd_mod scsi_mod
 CPU: 16 PID: 226 Comm: kworker/u241:4 Not tainted 4.1.20 #1
 Hardware name: Dell Inc. PowerEdge C6220/04GD66, BIOS 2.2.3 11/07/2013
 Workqueue: events_unbound async_run_entry_fn
  0000000000000000 ffff880859a5baf8 ffffffff81502872 ffff880859a5bb48
  0000000000000009 ffff880859a5bb38 ffffffff810692a5 ffff880859ee8828
  ffffffff812ad02c ffff880859d58320 ffffffff81abfdb0 ffff880859eb90c0
 Call Trace:
  [<ffffffff81502872>] dump_stack+0x4d/0x63
  [<ffffffff810692a5>] warn_slowpath_common+0xa1/0xbb
  [<ffffffff812ad02c>] ? __list_add+0x3c/0xa9
  [<ffffffff81069305>] warn_slowpath_fmt+0x46/0x48
  [<ffffffff812ad02c>] __list_add+0x3c/0xa9
  [<ffffffff81406f28>] md_autodetect_dev+0x41/0x62
  [<ffffffff81285862>] rescan_partitions+0x25f/0x29d
  [<ffffffff81506372>] ? mutex_lock+0x13/0x31
  [<ffffffff811a090f>] __blkdev_get+0x1aa/0x3cd
  [<ffffffff811a0b91>] blkdev_get+0x5f/0x294
  [<ffffffff81377ceb>] ? put_device+0x17/0x19
  [<ffffffff8128227c>] ? disk_put_part+0x12/0x14
  [<ffffffff812836f3>] add_disk+0x29d/0x407
  [<ffffffff81384345>] ? __pm_runtime_use_autosuspend+0x5c/0x64
  [<ffffffffa004a724>] sd_probe_async+0x115/0x1af [sd_mod]
  [<ffffffff81083177>] async_run_entry_fn+0x72/0x12c
  [<ffffffff8107c44c>] process_one_work+0x198/0x2ce
  [<ffffffff8107cac7>] worker_thread+0x1dd/0x2bb
  [<ffffffff8107c8ea>] ? cancel_delayed_work_sync+0x15/0x15
  [<ffffffff8107c8ea>] ? cancel_delayed_work_sync+0x15/0x15
  [<ffffffff81080d9c>] kthread+0xae/0xb6
  [<ffffffff81080000>] ? param_array_set+0x40/0xfa
  [<ffffffff81080cee>] ? __kthread_parkme+0x61/0x61
  [<ffffffff81508152>] ret_from_fork+0x42/0x70
  [<ffffffff81080cee>] ? __kthread_parkme+0x61/0x61

I suspect it is because there is no lock protecting this
global list, autostart_arrays() is called in ioctl() path
where there is no lock.

Cc: Shaohua Li <shli@kernel.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Shaohua Li <shli@fb.com>