selftests/bpf: add bpf_call test

strip always_inline from test_l4lb.c and compile it with -fno-inline
to let verifier go through 11 function with various function arguments
and return values

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>