selftests/bpf: Update xdp_context_test_run test to check maximum metadata size

selftests/bpf: Update xdp_context_test_run test to check maximum metadata size

Update the selftest to check that the metadata size check takes the
xdp_frame size into account in bpf_prog_test_run. The original
check (for meta size 256) was broken because the data frame supplied was
smaller than this, triggering a different EINVAL return. So supply a
larger data frame for this test to make sure we actually exercise the
check we think we are.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260105114747.1358750-2-toke@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>