configs: enable null block device for blk-mq tests

to #26648913
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

configs/config-4.19.y-x86_64

@@ -1660,7 +1660,7 @@ CONFIG_PNP=y
 #
 CONFIG_PNPACPI=y
 CONFIG_BLK_DEV=y
-# CONFIG_BLK_DEV_NULL_BLK is not set
+CONFIG_BLK_DEV_NULL_BLK=m
 # CONFIG_BLK_DEV_PCIESSD_MTIP32XX is not set
 CONFIG_ZRAM=m
 CONFIG_ZRAM_WRITEBACK=y