selftests/bpf: Add struct argument tests with fentry/fexit programs.

Add various struct argument tests with fentry/fexit programs.
Also add one test with a kernel func which does not have any
argument to test BPF_PROG2 macro in such situation.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220831152713.2080039-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>