btrfs: use the assigned fs_devices instead of the dereference

We have assigned the %fs_info->fs_devices in %fs_devices as its not
modified just use it for the mutex_lock().

Signed-off-by: Anand Jain <anand.jain@oracle.com>

fs/btrfs/volumes.c

@@ -2201,7 +2201,7 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
  INIT_LIST_HEAD(&seed_devices->alloc_list);
  mutex_init(&seed_devices->device_list_mutex);
 
- mutex_lock(&fs_info->fs_devices->device_list_mutex);
+ mutex_lock(&fs_devices->device_list_mutex);
  list_splice_init_rcu(&fs_devices->devices, &seed_devices->devices,
  synchronize_rcu);
  list_for_each_entry(device, &seed_devices->devices, dev_list)
@@ -2221,7 +2221,7 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
  generate_random_uuid(fs_devices->fsid);
  memcpy(fs_info->fsid, fs_devices->fsid, BTRFS_FSID_SIZE);
  memcpy(disk_super->fsid, fs_devices->fsid, BTRFS_FSID_SIZE);
- mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+ mutex_unlock(&fs_devices->device_list_mutex);
 
  super_flags = btrfs_super_flags(disk_super) &
  ~BTRFS_SUPER_FLAG_SEEDING;