netlink: don't bail out on dump errors

netlink: don't bail out on dump errors

This patch removes the existing error messages on netlink dump errors.
These functions used to be called from list commands. These days they
are called from the cache cache population path.

Note that nft breaks with older kernels at netlink_list_objs() since we
have no stateful objects support there.

Silence errors at this stage and return an empty list, thus, nft bails
out on explicit user commands if no nf_tables support is available.

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