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>

drivers/block/Kconfig

@@ -15,6 +15,9 @@ menuconfig BLK_DEV
 
 if BLK_DEV
 
+config BLK_DEV_NULL_BLK
+	tristate "Null test block driver"
+
 config BLK_DEV_FD
 	tristate "Normal floppy disk support"
 	depends on ARCH_MAY_HAVE_PC_FDC

drivers/block/Makefile

@@ -41,6 +41,7 @@ obj-$(CONFIG_BLK_DEV_RBD)     += rbd.o
 obj-$(CONFIG_BLK_DEV_PCIESSD_MTIP32XX)	+= mtip32xx/
 
 obj-$(CONFIG_BLK_DEV_RSXX) += rsxx/
+obj-$(CONFIG_BLK_DEV_NULL_BLK)	+= null_blk.o
 
 nvme-y		:= nvme-core.o nvme-scsi.o
 swim_mod-y	:= swim.o swim_asm.o