null_blk: Move driver into its own directory

[ Upstream commit eebf34a8 ]

Move null_blk driver code into the new sub-directory
drivers/block/null_blk.

Suggested-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: 63f88659 ("block: null_blk: Fix handling of fake timeout request")
Signed-off-by: Sasha Levin <sashal@kernel.org>