selftest/bpf: Adjust expected verifier errors

The verifier errors around stack accesses have changed slightly in the
previous commit (generally for the better).

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210207011027.676572-3-andreimatei1@gmail.com