libbpf: Make DECLARE_LIBBPF_OPTS macro strictly a variable declaration

LIBBPF_OPTS is implemented as a mix of field declaration and memset
+ assignment. This makes it neither variable declaration nor purely
statements, which is a problem, because you can't mix it with either
other variable declarations nor other function statements, because C90
compiler mode emits warning on mixing all that together.

This patch changes LIBBPF_OPTS into a strictly declaration of variable
and solves this problem, as can be seen in case of bpftool, which
previously would emit compiler warning, if done this way (LIBBPF_OPTS as
part of function variables declaration block).

This patch also renames LIBBPF_OPTS into DECLARE_LIBBPF_OPTS to follow
kernel convention for similar macros more closely.

v1->v2:
- rename LIBBPF_OPTS into DECLARE_LIBBPF_OPTS (Jakub Sitnicki).

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20191022172100.3281465-1-andriin@fb.com

tools/testing/selftests/bpf/prog_tests/attach_probe.c

@@ -50,7 +50,7 @@ void test_attach_probe(void)
 	const int kprobe_idx = 0, kretprobe_idx = 1;
 	const int uprobe_idx = 2, uretprobe_idx = 3;
 	const char *obj_name = "attach_probe";
-	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, open_opts,
 		.object_name = obj_name,
 		.relaxed_maps = true,
 	);

tools/testing/selftests/bpf/prog_tests/core_reloc.c

@@ -377,7 +377,7 @@ void test_core_reloc(void)
 		if (!test__start_subtest(test_case->case_name))
 			continue;
 
-		LIBBPF_OPTS(bpf_object_open_opts, opts,
+		DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
 			.relaxed_core_relocs = test_case->relaxed_core_relocs,
 		);
 

tools/testing/selftests/bpf/prog_tests/reference_tracking.c

@@ -5,7 +5,7 @@ void test_reference_tracking(void)
 {
 	const char *file = "test_sk_lookup_kern.o";
 	const char *obj_name = "ref_track";
-	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, open_opts,
 		.object_name = obj_name,
 		.relaxed_maps = true,
 	);