loop: Push loop_ctl_mutex down to loop_change_fd()

commit c371077000f4138ee3c15fbed50101ff24bdc91d upstream.

Push loop_ctl_mutex down to loop_change_fd(). We will need this to be
able to call loop_reread_partitions() without loop_ctl_mutex.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>