selftests/bpf: Free per-cpu values array in bpf_iter selftest

Array holding per-cpu values wasn't freed. Fix that.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211107165521.9240-5-andrii@kernel.org