selftests/bpf: fix compiler warnings in RELEASE=1 mode

When compiling BPF selftests with RELEASE=1, we get two new
warnings, which are treated as errors. Fix them.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20231212225343.1723081-1-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>