basic04: Switch back to old-style map definition

While updating libbpf, I also accidentally committed a change to the
basic04 lesson that started using the new btf-based map definition syntax.
This doesn't work with LLVM versions before v9, so let's revert this change
until we decide to require LLVM9 for the whole tutorial.

Fixes #85.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo

basic04-pinning-maps/xdp_prog_kern.c

@@ -8,13 +8,12 @@
  * - Here an array with XDP_ACTION_MAX (max_)entries are created.
  * - The idea is to keep stats per (enum) xdp_action
  */
-struct {
-	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
-	__uint(max_entries, XDP_ACTION_MAX);
-	__type(key, __u32);
-	__type(value, struct datarec);
-	__uint(pinning, 1);
-} xdp_stats_map SEC(".maps");
+struct bpf_map_def SEC("maps") xdp_stats_map = {
+	.type        = BPF_MAP_TYPE_PERCPU_ARRAY,
+	.key_size    = sizeof(__u32),
+	.value_size  = sizeof(struct datarec),
+	.max_entries = XDP_ACTION_MAX,
+};
 
 /* LLVM maps __sync_fetch_and_add() as a built-in function to the BPF atomic add
  * instruction (that is BPF_STX | BPF_XADD | BPF_W for word sizes)