selftests/bpf: Add BPF XDP link selftests

Add selftest validating all the attachment logic around BPF XDP link. Test
also link updates and get_obj_info() APIs.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722064603.3350758-9-andriin@fb.com

tools/testing/selftests/bpf/progs/test_xdp_link.c

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+char LICENSE[] SEC("license") = "GPL";
+
+SEC("xdp/handler")
+int xdp_handler(struct xdp_md *xdp)
+{
+	return 0;
+}