selftests/bpf: Add flags and VLAN hint to xdp_hw_metadata

Add VLAN hint to the xdp_hw_metadata program.

Also, to make metadata layout more straightforward, add flags field
to pass information about validity of every separate hint separately.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/r/20231205210847.28460-17-larysa.zaremba@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>