selftests/bpf: fix tests due to const spill/fill

commit fc559a70d57c6ee5443f7a750858503e94cdc941 upstream.

fix tests that incorrectly assumed that the verifier
cannot track constants through stack.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
[OP: backport to 4.19]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>