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/lib/bpf/libbpf.c

@@ -3678,7 +3678,7 @@ out:
 static struct bpf_object *
 __bpf_object__open_xattr(struct bpf_object_open_attr *attr, int flags)
 {
-	LIBBPF_OPTS(bpf_object_open_opts, opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
 		.relaxed_maps = flags & MAPS_RELAX_COMPAT,
 	);
 
@@ -3730,7 +3730,7 @@ struct bpf_object *
 bpf_object__open_buffer(const void *obj_buf, size_t obj_buf_sz,
 			const char *name)
 {
-	LIBBPF_OPTS(bpf_object_open_opts, opts,
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts,
 		.object_name = name,
 		/* wrong default, but backwards-compatible */
 		.relaxed_maps = true,