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