mmc: core: Prevent bus reference leak in mmc_blk_init()

Upon module load, mmc_block allocates a bus with bus_registeri() in
mmc_blk_init(). This reference never gets freed during module unload, which
leads to subsequent re-insertions of the module fails and a WARN() splat is
triggered.

Fix the bug by dropping the reference for the bus in mmc_blk_exit().

Signed-off-by: Alexander Kappner <agk@godking.net>
Fixes: 97548575be ("mmc: block: Convert RPMB to a character device")
Cc: <stable@vger.kernel.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/block.c

@@ -3080,6 +3080,7 @@ static void __exit mmc_blk_exit(void)
 	mmc_unregister_driver(&mmc_driver);
 	unregister_blkdev(MMC_BLOCK_MAJOR, "mmc");
 	unregister_chrdev_region(mmc_rpmb_devt, MAX_DEVICES);
+	bus_unregister(&mmc_rpmb_bus_type);
 }
 
 module_init(mmc_blk_init);