ext4: fix missing iput of root inode for some mount error paths

This assures that the root inode is not leaked, and that sb->s_root is
NULL, which will prevent generic_shutdown_super() from doing extra
work, including call sync_filesystem, which ultimately results in
ext4_sync_fs() getting called with an uninitialized struct super,
which is the cause of the crash noted in Kernel Bugzilla #26752.

https://bugzilla.kernel.org/show_bug.cgi?id=26752Signed-off-by: Manish Katiyar <mkatiyar@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -3521,17 +3521,16 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	if (IS_ERR(root)) {
 		ext4_msg(sb, KERN_ERR, "get root inode failed");
 		ret = PTR_ERR(root);
+		root = NULL;
 		goto failed_mount4;
 	}
 	if (!S_ISDIR(root->i_mode) || !root->i_blocks || !root->i_size) {
-		iput(root);
 		ext4_msg(sb, KERN_ERR, "corrupt root inode, run e2fsck");
 		goto failed_mount4;
 	}
 	sb->s_root = d_alloc_root(root);
 	if (!sb->s_root) {
 		ext4_msg(sb, KERN_ERR, "get root dentry failed");
-		iput(root);
 		ret = -ENOMEM;
 		goto failed_mount4;
 	}
@@ -3647,6 +3646,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	goto failed_mount;
 
 failed_mount4:
+	iput(root);
+	sb->s_root = NULL;
 	ext4_msg(sb, KERN_ERR, "mount failed");
 	destroy_workqueue(EXT4_SB(sb)->dio_unwritten_wq);
 failed_mount_wq: