selftests/bpf: Fix test dynptr/test_dynptr_copy_xdp failure

For arm64 64K page size, the bpf_dynptr_copy() in test dynptr/test_dynptr_copy_xdp
will succeed, but the test will failure with 4K page size. This patch made a change
so the test will fail expectedly for both 4K and 64K page sizes.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://patch.msgid.link/20250725043435.208974-1-yonghong.song@linux.dev