libbpf: disassociate section handler on explicit bpf_program__set_type() call

If user explicitly overrides programs's type with
bpf_program__set_type() API call, we need to disassociate whatever
SEC_DEF handler libbpf determined initially based on program's SEC()
definition, as it's not goind to be valid anymore and could lead to
crashes and/or confusing failures.

Also, fix up bpf_prog_test_load() helper in selftests/bpf, which is
force-setting program type (even if that's completely unnecessary; this
is quite a legacy piece of code), and thus should expect auto-attach to
not work, yet one of the tests explicitly relies on auto-attach for
testing.

Instead, force-set program type only if it differs from the desired one.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230327185202.1929145-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Müller <deso@posteo.net>

src/libbpf.c

@@ -8468,6 +8468,7 @@ int bpf_program__set_type(struct bpf_program *prog, enum bpf_prog_type type)
 		return libbpf_err(-EBUSY);
 
 	prog->type = type;
+	prog->sec_def = NULL;
 	return 0;
 }