[NETFILTER] ctnetlink: Fix refcount leak ip_conntrack/nat_proto

Remove proto == NULL checking since ip_conntrack_[nat_]proto_find_get
always returns a valid pointer.

Fix missing ip_conntrack_proto_put in some paths.

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