libbpf: Fix bpf_object name determination for bpf_object__open_file()

If bpf_object__open_file() gets path like "some/dir/obj.o", it should derive
BPF object's name as "obj" (unless overriden through opts->object_name).
Instead, due to using `path` as a fallback value for opts->obj_name, path is
used as is for object name, so for above example BPF object's name will be
verbatim "some/dir/obj", which leads to all sorts of troubles, especially when
internal maps are concern (they are using up to 8 characters of object name).
Fix that by ensuring object_name stays NULL, unless overriden.

Fixes: 291ee02 ("libbpf: Refactor bpf_object__open APIs to use common opts")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191122003527.551556-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -3940,7 +3940,7 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 	if (!OPTS_VALID(opts, bpf_object_open_opts))
 		return ERR_PTR(-EINVAL);
 
-	obj_name = OPTS_GET(opts, object_name, path);
+	obj_name = OPTS_GET(opts, object_name, NULL);
 	if (obj_buf) {
 		if (!obj_name) {
 			snprintf(tmp_name, sizeof(tmp_name), "%lx-%lx",