null_blk: register as a LightNVM device

Add support for registering as a LightNVM device. This allows us to
evaluate the performance of the LightNVM subsystem.

In /drivers/Makefile, LightNVM is moved above block device drivers
to make sure that the LightNVM media managers have been initialized
before drivers under /drivers/block are initialized.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Fix by Jens Axboe to remove unneeded slab cache and the following
memory leak.
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/Makefile

@@ -63,6 +63,7 @@ obj-$(CONFIG_FB_I810)           += video/fbdev/i810/
 obj-$(CONFIG_FB_INTEL)          += video/fbdev/intelfb/
 
 obj-$(CONFIG_PARPORT)		+= parport/
+obj-$(CONFIG_NVM)		+= lightnvm/
 obj-y				+= base/ block/ misc/ mfd/ nfc/
 obj-$(CONFIG_LIBNVDIMM)		+= nvdimm/
 obj-$(CONFIG_DMA_SHARED_BUFFER) += dma-buf/
@@ -70,7 +71,6 @@ obj-$(CONFIG_NUBUS)		+= nubus/
 obj-y				+= macintosh/
 obj-$(CONFIG_IDE)		+= ide/
 obj-$(CONFIG_SCSI)		+= scsi/
-obj-$(CONFIG_NVM)		+= lightnvm/
 obj-y				+= nvme/
 obj-$(CONFIG_ATA)		+= ata/
 obj-$(CONFIG_TARGET_CORE)	+= target/