rule: reset cache iff there is an existing cache

rule: reset cache iff there is an existing cache

If genid is unset, then do not reset existing cache. In the kernel,
generation ID is assumed to be always != zero.

This patch fixes:

	nft 'add table x; add chain x y;'

that allow us to send several commands in one single batch from the
command line.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>