selftests/bpf: Increase invalid metadata size

Changed check expects passed data meta to be deemed invalid. After loosening
the requirement, the size of 36 bytes becomes valid. Therefore, increase
tested meta size to 256, so we do not get an unexpected success.

Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231206205919.404415-2-larysa.zaremba@intel.com