* Demonstration of close() hang on bpf_link fd close

This is a minimal example to demonstrate an issue where =close()='ing an fd for a
bpf_link kernel structure will hang indefinitely if the system is fully loaded.
This only happens on a kernel compiled with =CONFIG_PREEMPT=y=, but on such a
kernel the issue is quite easy to trigger: Simply load up the system (with work
inside the kernel; sending it a lot of network traffic works pretty well), and
run the bpf-link-hang program as root. The program will hang until load is
lowered...

The hang happens in =synchronize_rcu_tasks()=, which can be seen with this
bpftrace invocation:

#+begin_src sh
bpftrace -e 'kprobe:synchronize_rcu_tasks { @start = nsecs; printf("enter\n"); } kretprobe:synchronize_rcu_tasks { printf("exit after %d ms\n", (nsecs - @start) / 1000000); }'
#+end_src