netfilter: ctnetlink: remove incorrect spin_[un]lock_bh on NAT module autoload

Since 7d367e06, ctnetlink_new_conntrack is called without holding
the nf_conntrack_lock spinlock. Thus, ctnetlink_parse_nat_setup
does not require to release that spinlock anymore in the NAT module
autoload case.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>