selftests/bpf: Fix task_local_data failure with 64K page

selftests/bpf: Fix task_local_data failure with 64K page

On arm64 systems with 64K pages, the selftest task_local_data has the following
failures:
  ...
  test_task_local_data_basic:PASS:tld_create_key 0 nsec
  test_task_local_data_basic:FAIL:tld_create_key unexpected tld_create_key: actual 0 != expected -28
  ...
  test_task_local_data_basic_thread:PASS:run task_main 0 nsec
  test_task_local_data_basic_thread:FAIL:task_main retval unexpected error: 2 (errno 0)
  test_task_local_data_basic_thread:FAIL:tld_get_data value0 unexpected tld_get_data value0: actual 0 != expected 6268
  ...
  #447/1   task_local_data/task_local_data_basic:FAIL
  ...
  #447/2   task_local_data/task_local_data_race:FAIL
  #447     task_local_data:FAIL

When TLD_DYN_DATA_SIZE is 64K page size, for
  struct tld_meta_u {
       _Atomic __u8 cnt;
       __u16 size;
        struct tld_metadata metadata[];
  };
field 'cnt' would overflow. For example, for 4K page, 'cnt' will
be 4096/64 = 64. But for 64K page, 'cnt' will be 65536/64 = 1024
and 'cnt' is not enough for 1024. To accommodate 64K page,
'_Atomic __u8 cnt' becomes '_Atomic __u16 cnt'. A few other places
are adjusted accordingly.

In test_task_local_data.c, the value for TLD_DYN_DATA_SIZE is changed
from 4096 to (getpagesize() - 8) since the maximum buffer size for
TLD_DYN_DATA_SIZE is (getpagesize() - 8).

Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Cc: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260123055122.494352-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>