null_blk: multi queue aware block test driver

A driver that simply completes IO it receives, it does no
transfers. Written to fascilitate testing of the blk-mq code.
It supports various module options to use either bio queueing,
rq queueing, or mq mode.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --cc drivers/block/Kconfig
Simple merge

diff --cc drivers/block/Makefile
Simple merge

diff --cc drivers/block/null_blk.c
Simple merge