selftests/bpf: Fix strict mode calculation

"(__LIBBPF_STRICT_LAST - 1) & ~LIBBPF_STRICT_MAP_DEFINITIONS" is wrong
as it is equal to 0 (LIBBPF_STRICT_NONE). Let's use
"LIBBPF_STRICT_ALL & ~LIBBPF_STRICT_MAP_DEFINITIONS" now that the
previous commit makes it possible in libbpf.

Fixes: 93b8952d ("libbpf: deprecate legacy BPF map definitions")
Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220207145052.124421-4-mauricio@kinvolk.io