selftests/bpf: Fix cgroup sockopt verifier test

Since the BPF_PROG_TYPE_CGROUP_SOCKOPT verifier test does not set an
attach type, bpf_prog_load_check_attach() disallows loading the program
and the test is always skipped:

 torvalds#434/p perfevent for cgroup sockopt SKIP (unsupported program type 25)

Fix the issue by setting a valid attach type.

Fixes: 0456ea1 ("bpf: Enable more helpers for BPF_PROG_TYPE_CGROUP_{DEVICE,SYSCTL,SOCKOPT}")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20200710150439.126627-1-jean-philippe@linaro.org

tools/testing/selftests/bpf/verifier/event_output.c

@@ -112,6 +112,7 @@
 	"perfevent for cgroup sockopt",
 	.insns =  { __PERF_EVENT_INSNS__ },
 	.prog_type = BPF_PROG_TYPE_CGROUP_SOCKOPT,
+	.expected_attach_type = BPF_CGROUP_SETSOCKOPT,
 	.fixup_map_event_output = { 4 },
 	.result = ACCEPT,
 	.retval = 1,