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:

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

Fix the issue by setting a valid attach type.

Fixes: 0456ea170cd6 ("bpf: Enable more helpers for BPF_PROG_TYPE_CGROUP_{DEVICE,SYSCTL,SOCKOPT}")
Signed-off-by: Jean-Philippe Brucker <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Jakub Sitnicki <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 	"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,