netfilter: ipset: rework cidr bookkeeping

netfilter: ipset: rework cidr bookkeeping

According to sashiko, the current bookkeeping of cidr values are unsafe
on weakly-ordered architectures. Replace the in-place updating with an
RCU based method: create the new bookeeping structure, update and replace
the old one with the new. Downside that we need to allocate memory when
deleting a cidr entry - in case of memory pressure fall back to leave holes
which possibility is taken into account at evaluation time.

Thanks to Pablo (Pablo Neira Ayuso <pablo@netfilter.org>) and Cyntia
(Cynthia <cynthia@kosmx.dev>) for helping me in debugging which resulted
the patch "netfilter: ipset: allocate the proper memory for the generic
hash structure" on which this very patch depends.

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