netfilter: xtables: consistent struct compat_xt_counters definition

There is compat_u64 type which deals with different u64 type alignment
on different compat-capable platforms, so use it and removed some
hardcoded assumptions.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>