debugfs: allow bad parent pointers to be passed in

If something went wrong with creating a debugfs file/symlink/directory,
that value could be passed down into debugfs again as a parent dentry.
To make caller code simpler, just error out if this happens, and don't
crash the kernel.
Reported-by: NAlex Elder <elder@linaro.org>
Reviewed-by: NViresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: NAlex Elder <elder@linaro.org>

fs/debugfs/inode.c

@@ -254,6 +254,9 @@ static struct dentry *start_creating(const char *name, struct dentry *parent)
 
 	pr_debug("debugfs: creating file '%s'\n",name);
 
+	if (IS_ERR(parent))
+		return parent;
+
 	error = simple_pin_fs(&debug_fs_type, &debugfs_mount,
 			      &debugfs_mount_count);
 	if (error)