nefilter: eache: reduce struct size from 32 to 24 byte

Only "cache" needs to use ulong (its used with set_bit()), missed can use
u16.  Also add build-time assertion to ensure event bits fit.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>