md: Explicitly create command-line configured devices

Boot-time assembly of arrays with md= command-line arguments breaks when
CONFIG_BLOCK_LEGACY_AUTOLOAD is unset. md_setup_drive() in md-autodetect.c
calls blkdev_get_by_dev(), assuming this implicitly creates the block
device.

Fix this by attempting to md_alloc() the array first. As in the probe path,
ignore any error as failure is caught by blkdev_get_by_dev() anyway.
Signed-off-by: NChris Webb <chris@arachsys.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NSong Liu <song@kernel.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/md/md-autodetect.c

@@ -169,6 +169,7 @@ static void __init md_setup_drive(struct md_setup_args *args)
 
 	pr_info("md: Loading %s: %s\n", name, args->device_names);
 
+	md_alloc(mdev, name);
 	bdev = blkdev_get_by_dev(mdev, FMODE_READ, NULL);
 	if (IS_ERR(bdev)) {
 		pr_err("md: open failed - cannot start array %s\n", name);

drivers/md/md.c

@@ -5623,7 +5623,7 @@ int mddev_init_writes_pending(struct mddev *mddev)
 }
 EXPORT_SYMBOL_GPL(mddev_init_writes_pending);
 
-static int md_alloc(dev_t dev, char *name)
+int md_alloc(dev_t dev, char *name)
 {
 	/*
 	 * If dev is zero, name is the name of a device to allocate with

drivers/md/md.h

@@ -765,6 +765,7 @@ extern int md_integrity_add_rdev(struct md_rdev *rdev, struct mddev *mddev);
 extern int strict_strtoul_scaled(const char *cp, unsigned long *res, int scale);
 
 extern void mddev_init(struct mddev *mddev);
+int md_alloc(dev_t dev, char *name);
 extern int md_run(struct mddev *mddev);
 extern int md_start(struct mddev *mddev);
 extern void md_stop(struct mddev *mddev);