datapath: Detect upstream nf_nat change

datapath: Detect upstream nf_nat change

The following two upstream commits merge nf_nat_ipv4 and nf_nat_ipv6
into nf_nat core, and move some header files around.  To handle
these modifications, this patch detects the upstream changes, uses
the header files and config symbols properly.

Ideally, we should replace CONFIG_NF_NAT_IPV4 and CONFIG_NF_NAT_IPV6 with
CONFIG_NF_NAT and CONFIG_IPV6.  In order to keep backward compatibility,
we keep the checking of CONFIG_NF_NAT_IPV4/6 as is for the old kernel,
and replace them with marco for the new kernel.

upstream commits:
3bf195ae6037 ("netfilter: nat: merge nf_nat_ipv4,6 into nat core")
d2c5c103b133 ("netfilter: nat: remove nf_nat_l3proto.h and nf_nat_core.h")

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>