btrfs: fix lock dep warning move scratch super outside of chunk_mutex

btrfs: fix lock dep warning move scratch super outside of chunk_mutex

commit 48b3b9d401ec86899a52003b37331190a35a81a6 upstream.

Move scratch super outside of the chunk lock to avoid below
lockdep warning. The better place to scratch super is in
the function btrfs_rm_dev_replace_free_srcdev() just before
free_device, which is outside of the chunk lock as well.

To reproduce:
  (fresh boot)
  mkfs.btrfs -f -draid5 -mraid5 /dev/sdc /dev/sdd /dev/sde
  mount /dev/sdc /btrfs
  dd if=/dev/zero of=/btrfs/tf1 bs=4096 count=100
  (get devmgt from https://github.com/asj/devmgt.git)
  devmgt detach /dev/sde
  dd if=/dev/zero of=/btrfs/tf1 bs=4096 count=100
  sync
  btrfs replace start -Brf 3 /dev/sdf /btrfs <--
  devmgt attach host7

======================================================
[ INFO: possible circular locking dependency detected ]
4.6.0-rc2asj+ #1 Not tainted
---------------------------------------------------

btrfs/2174 is trying to acquire lock:
(sb_writers){.+.+.+}, at:
[<ffffffff812449b4>] __sb_start_write+0xb4/0xf0

but task is already holding lock:
(&fs_info->chunk_mutex){+.+.+.}, at:
[<ffffffffa05c5f55>] btrfs_dev_replace_finishing+0x145/0x980 [btrfs]

which lock already depends on the new lock.

Chain exists of:
sb_writers --> &fs_devs->device_list_mutex --> &fs_info->chunk_mutex
Possible unsafe locking scenario:
CPU0				CPU1
----				----
lock(&fs_info->chunk_mutex);
				lock(&fs_devs->device_list_mutex);
				lock(&fs_info->chunk_mutex);
lock(sb_writers);