loop: fix to a race condition due to the early registration of device

The early device registration made possible a race leading to allocations
of disks with wrong minors.

This patch moves the device registration further down the loop_init
function to make the race infeasible.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Volkov <avolkov@ispras.ru>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>