selftests/bpf: use preferred setter/getter APIs instead of deprecated ones

Switch to using preferred setters and getters instead of deprecated ones.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220124194254.2051434-6-andrii@kernel.org


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