[NETFILTER] Free layer-3 specific protocol tables at cleanup

Although the comment around the allocation code tells us that
the layer-3 specific protocol tables will be freed when cleaning up,
they aren't. And this makes nfsim complain loudly...

Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>