selftests/bpf: Increase xdp data size for arm64 64K page size

With arm64 64K page size, the following 4 subtests failed:
  #97/25   dynptr/test_probe_read_user_dynptr:FAIL
  #97/26   dynptr/test_probe_read_kernel_dynptr:FAIL
  #97/27   dynptr/test_probe_read_user_str_dynptr:FAIL
  #97/28   dynptr/test_probe_read_kernel_str_dynptr:FAIL

These failures are due to function bpf_dynptr_check_off_len() in
include/linux/bpf.h where there is a test
  if (len > size || offset > size - len)
    return -E2BIG;
With 64K page size, the 'offset' is greater than 'size - len',
which caused the test failure.

For 64KB page size, this patch increased the xdp buffer size from 5000 to
90000. The above 4 test failures are fixed as 'size' value is increased.
But it introduced two new failures:
  #97/4    dynptr/test_dynptr_copy_xdp:FAIL
  #97/12   dynptr/test_dynptr_memset_xdp_chunks:FAIL

These two failures will be addressed in subsequent patches.

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/20250725043430.208469-1-yonghong.song@linux.dev