userns and mnt_idmap leak in open_tree_attr(2)

In the Linux kernel, the following vulnerability has been resolved:

userns and mnt_idmap leak in open_tree_attr(2)

Once want_mount_setattr() has returned a positive, it does require

finish_mount_kattr() to release ->mnt_userns. Failing do_mount_setattr()

does not change that.

As the result, we can end up leaking userns and possibly mnt_idmap as

well.