selftests/bpf: Assert link info uprobe_multi count & path_size if unset

Add assertions in `bpf_link_info.uprobe_multi` test to verify that
`count` and `path_size` fields are correctly populated when the fields
are unset.

This tests a previous bug where the `path_size` field was not populated
when `path` and `path_size` were unset.

Signed-off-by: Tyrone Wu <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 	if (!ASSERT_NEQ(err, -1, "readlink"))
 		return -1;
 
+	memset(&info, 0, sizeof(info));
+	err = bpf_link_get_info_by_fd(fd, &info, &len);
+	if (!ASSERT_OK(err, "bpf_link_get_info_by_fd"))
+		return -1;
+
+	ASSERT_EQ(info.uprobe_multi.count, 3, "info.uprobe_multi.count");
+	ASSERT_EQ(info.uprobe_multi.path_size, strlen(path) + 1,
+		  "info.uprobe_multi.path_size");
+
 	for (bit = 0; bit < 8; bit++) {
 		memset(&info, 0, sizeof(info));
 		info.uprobe_multi.path = ptr_to_u64(path_buf);