null_blk: add usage hints for NVM

If CONFIG_NVM is disabled, loading null_block module with use_lightnvm=1
fails. But there are no messages and documents related to the failure.

Add the appropriate error message.
Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

Massaged the text a bit.
Signed-off-by: Jens Axboe <axboe@fb.com>

Documentation/block/null_blk.txt

@@ -72,4 +72,4 @@ use_per_node_hctx=[0/1]: Default: 0
      queue for each CPU node in the system.
 
 use_lightnvm=[0/1]: Default: 0
-  Register device with LightNVM. Requires blk-mq to be used.
+  Register device with LightNVM. Requires blk-mq and CONFIG_NVM to be enabled.

drivers/block/null_blk.c

@@ -577,6 +577,7 @@ static void null_nvm_unregister(struct nullb *nullb)
 #else
 static int null_nvm_register(struct nullb *nullb)
 {
+	pr_err("null_blk: CONFIG_NVM needs to be enabled for LightNVM\n");
 	return -EINVAL;
 }
 static void null_nvm_unregister(struct nullb *nullb) {}