[S390] hypfs crashes with invalid mount option.

When an invalid mount option is specified, no root inode is created
for hypfs, hypfs_fill_super() returns with -EINVAL and then
hypfs_kill_super() is called. hypfs_kill_super() does not check if
the root inode has been initialized. This patch adds this check.
Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/hypfs/inode.c

@@ -312,10 +312,12 @@ static void hypfs_kill_super(struct super_block *sb)
 {
 	struct hypfs_sb_info *sb_info = sb->s_fs_info;
 
-	hypfs_delete_tree(sb->s_root);
-	hypfs_remove(sb_info->update_file);
-	kfree(sb->s_fs_info);
-	sb->s_fs_info = NULL;
+	if (sb->s_root) {
+		hypfs_delete_tree(sb->s_root);
+		hypfs_remove(sb_info->update_file);
+		kfree(sb->s_fs_info);
+		sb->s_fs_info = NULL;
+	}
 	kill_litter_super(sb);
 }