selftest/bpf: Add a recursion test

Add recursive non-sleepable fentry program as a test.
All attach points where sleepable progs can execute are non recursive so far.
The recursion protection mechanism for sleepable cannot be activated yet.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210210033634.62081-6-alexei.starovoitov@gmail.com