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

stable inclusion
from linux-4.19.207
commit fc578c64398df4f93fd7a6143e218281cc7c4348

--------------------------------

commit fc559a70 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>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>