rust_binderfs: fix a dentry leak

rust_binderfs: fix a dentry leak

Parallel to binderfs patches - 02da8d2c0965 "binderfs_binder_ctl_create():
kill a bogus check" and the bit of b89aa544821d "convert binderfs" that
got lost when making 4433d8e25d73 "convert rust_binderfs"; the former is
a cleanup, the latter is about marking /binder-control persistent, so that
it would be taken out on umount.

Fixes: 4433d8e25d73 ("convert rust_binderfs")
Acked-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>