selftests/bpf: Make bpf_user_ringbuf_drain() selftest callback return 1

In commit 1bfe26fb ("bpf: Add verifier support for custom callback
return range"), the verifier was updated to require callbacks to BPF
helpers to explicitly specify the range of values that can be returned.
bpf_user_ringbuf_drain() was merged after this in commit 20571567
("bpf: Add bpf_user_ringbuf_drain() helper"), and this change in default
behavior was missed. This patch updates the BPF_MAP_TYPE_USER_RINGBUF
selftests to also return 1 from a bpf_user_ringbuf_drain() callback so
as to properly test this going forward.
Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221012232015.1510043-3-void@manifault.com

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

@@ -47,14 +47,14 @@ record_sample(struct bpf_dynptr *dynptr, void *context)
 		if (status) {
 			bpf_printk("bpf_dynptr_read() failed: %d\n", status);
 			err = 1;
-			return 0;
+			return 1;
 		}
 	} else {
 		sample = bpf_dynptr_data(dynptr, 0, sizeof(*sample));
 		if (!sample) {
 			bpf_printk("Unexpectedly failed to get sample\n");
 			err = 2;
-			return 0;
+			return 1;
 		}
 		stack_sample = *sample;
 	}