netfilter: nfnetlink: remove static declaration from err_list

Otherwise, different subsys will race to access the err_list, with holding
the different nfnl_lock(subsys_id).

But this will not happen now, since ->call_batch is only implemented by
nftables, so the err_list is protected by nfnl_lock(NFNL_SUBSYS_NFTABLES).

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>