ext4: simplify device handling

We removed all codepaths where s_umount is taken beneath open_mutex and
bd_holder_lock so don't make things more complicated than they need to
be and hold s_umount over block device opening.

Link: https://lore.kernel.org/r/20231024-vfs-super-rework-v1-3-37a8aa697148@kernel.orgReviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/ext4/super.c

@@ -5864,11 +5864,8 @@ static struct bdev_handle *ext4_get_journal_blkdev(struct super_block *sb,
 	struct ext4_super_block *es;
 	int errno;
 
-	/* see get_tree_bdev why this is needed and safe */
-	up_write(&sb->s_umount);
 	bdev_handle = bdev_open_by_dev(j_dev, BLK_OPEN_READ | BLK_OPEN_WRITE,
 				       sb, &fs_holder_ops);
-	down_write(&sb->s_umount);
 	if (IS_ERR(bdev_handle)) {
 		ext4_msg(sb, KERN_ERR,
 			 "failed to open journal device unknown-block(%u,%u) %ld",