Support calling kfuncs

[jfernandez]

Is your feature request related to a problem? Please describe.

There are some instrumentation use-cases where calling a kfunc would be useful. For example, when you want to get the cgroup struct from an ID using bpf_cgroup_from_id, or when you want to grab a task_struct when only a pid is available, via bpf_task_from_pid.

Today, I need to write my own BPF program to achieve this type of instrumentation.

Describe the solution you'd like

In BPF, you need to explicitly define which kfunc's your program will call, ie:

extern struct task_struct *bpf_task_from_pid(s32 pid);

bpftrace should also have something similar.

Describe alternative solutions or features you've considered

Writing my own bpf program.