selftests/bpf: Fix test dynptr/test_dynptr_memset_xdp_chunks failure

For arm64 64K page size, the xdp data size was set to be more than 64K
in one of previous patches. This will cause failure for bpf_dynptr_memset().
Since the failure of bpf_dynptr_memset() is expected with 64K page size,
return success.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250725043440.209266-1-yonghong.song@linux.dev