netfilter: xtables: symmetric COMPAT_XT_ALIGN definition

Rewrite COMPAT_XT_ALIGN in terms of dummy structure hack.
Compat counters logically have nothing to do with it.
Use ALIGN() macro while I'm at it for same types.

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