btrfs: use after free when closing devices

__btrfs_close_devices() would call_rcu to free the device, which is racy with
list_for_each_entry() accessing the memory to retrieve the next device on the
list.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>