selftests/bpf: Don't call fsopen() as privileged user

selftests/bpf: Don't call fsopen() as privileged user

In the BPF token example, the fsopen() syscall is called as privileged
user. This is unneeded because fsopen() can be called also as
unprivileged user from the user namespace.
As the `fs_fd` file descriptor which was sent back and forth is still the
same, keep it open instead of cloning and closing it twice via SCM_RIGHTS.

cfr. https://github.com/systemd/systemd/pull/36134

Signed-off-by: Matteo Croce <teknoraver@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/bpf/20250701183123.31781-1-technoboy85@gmail.com