netfilter: xt_limit: have r->cost != 0 case work

Commit v2.6.19-rc1~1272^2~41 tells us that r->cost != 0 can happen when
a running state is saved to userspace and then reinstated from there.

Make sure that private xt_limit area is initialized with correct values.
Otherwise, random matchings due to use of uninitialized memory.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>