netfilter: nf_tables: safe RCU iteration on list when dumping

The dump operation through netlink is not protected by the nfnl_lock.
Thus, a reader process can be dumping any of the existing object
lists while another process can be updating the list content.

This patch resolves this situation by protecting all the object
lists with RCU in the netlink dump path which is the reader side.
The updater path is already protected via nfnl_lock, so use list
manipulation RCU-safe operations.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>