null_blk: suppress invalid partition info

null_blk is partitionable, but it doesn't store any of the info. When
it is loaded, you would normally see:

[1226739.343608]  nullb0: unknown partition table
[1226739.343746]  nullb1: unknown partition table

which can confuse some people. Add the appropriate gendisk flag
to suppress this info.
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -579,7 +579,7 @@ static int null_add_dev(void)
 	sector_div(size, bs);
 	set_capacity(disk, size);
 
-	disk->flags |= GENHD_FL_EXT_DEVT;
+	disk->flags |= GENHD_FL_EXT_DEVT | GENHD_FL_SUPPRESS_PARTITION_INFO;
 	disk->major		= null_major;
 	disk->first_minor	= nullb->index;
 	disk->fops		= &null_fops;