null_blk: add blocking description and remove lightnvm

- The description of 'blocking' is missing in null_blk.txt

- The 'lightnvm' parameter has been removed in null_blk.c

This updates both in null_blk.txt.
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/null_blk.txt

@@ -71,13 +71,16 @@ use_per_node_hctx=[0/1]: Default: 0
   1: The multi-queue block layer is instantiated with a hardware dispatch
      queue for each CPU node in the system.
 
-use_lightnvm=[0/1]: Default: 0
-  Register device with LightNVM. Requires blk-mq and CONFIG_NVM to be enabled.
-
 no_sched=[0/1]: Default: 0
   0: nullb* use default blk-mq io scheduler.
   1: nullb* doesn't use io scheduler.
 
+blocking=[0/1]: Default: 0
+  0: Register as a non-blocking blk-mq driver device.
+  1: Register as a blocking blk-mq driver device, null_blk will set
+     the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
+     needs to block in its ->queue_rq() function.
+
 shared_tags=[0/1]: Default: 0
   0: Tag set is not shared.
   1: Tag set shared between devices for blk-mq. Only makes sense with