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

selftests/bpf: Add test for xdp_md context in BPF_PROG_TEST_RUN

Add a test for using xdp_md as a context to BPF_PROG_TEST_RUN for XDP
programs.

The test uses a BPF program that takes in a return value from XDP
meta data, then reduces the size of the XDP meta data by 4 bytes.

Test cases validate the possible failure cases for passing in invalid
xdp_md contexts, that the return value is successfully passed
in, and that the adjusted meta data is successfully copied out.

Co-developed-by: Cody Haas <chaas@riotgames.com>
Co-developed-by: Lisa Watanabe <lwatanabe@riotgames.com>
Signed-off-by: Cody Haas <chaas@riotgames.com>
Signed-off-by: Lisa Watanabe <lwatanabe@riotgames.com>
Signed-off-by: Zvi Effron <zeffron@riotgames.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210707221657.3985075-5-zeffron@riotgames.com