block: Rename the null_blk_mod kernel module back into null_blk

Commit ca4b2a011948 ("null_blk: add zone support") breaks several
blktests scripts because it renamed the null_blk kernel module into
null_blk_mod. Hence rename null_blk_mod back into null_blk.

Fixes: ca4b2a011948 ("null_blk: add zone support")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Matias Bjorling <matias.bjorling@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/Makefile

@@ -38,9 +38,9 @@ obj-$(CONFIG_BLK_DEV_PCIESSD_MTIP32XX)	+= mtip32xx/
 obj-$(CONFIG_BLK_DEV_RSXX) += rsxx/
 obj-$(CONFIG_ZRAM) += zram/
 
-obj-$(CONFIG_BLK_DEV_NULL_BLK)	+= null_blk_mod.o
-null_blk_mod-objs	:= null_blk.o
-null_blk_mod-$(CONFIG_BLK_DEV_ZONED) += null_blk_zoned.o
+obj-$(CONFIG_BLK_DEV_NULL_BLK)	+= null_blk.o
+null_blk-objs	:= null_blk_main.o
+null_blk-$(CONFIG_BLK_DEV_ZONED) += null_blk_zoned.o
 
 skd-y		:= skd_main.o
 swim_mod-y	:= swim.o swim_asm.o