btrfs: move volume_mutex into the btrfs_rm_device()

A cleanup patch no functional change, we hold volume_mutex before
calling btrfs_rm_device, so move it into the function itself.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>