datapath: NAT support for shifted portmap ranges

datapath: NAT support for shifted portmap ranges

This patch backports the following upstream commit from net-next, and
defines HAVE_NF_NAT_RANGE2 to determine whether to use
'struct nf_nat_range2'.

Upstream commit:
    commit 2eb0f624b709e78ec8e2f4c3412947703db99301
    Author: Thierry Du Tre <thierry@dtsystems.be>
    Date:   Wed Apr 4 15:38:22 2018 +0200

    netfilter: add NAT support for shifted portmap ranges

    This is a patch proposal to support shifted ranges in portmaps.  (i.e. tcp/udp
    incoming port 5000-5100 on WAN redirected to LAN 192.168.1.5:2000-2100)

    Currently DNAT only works for single port or identical port ranges.  (i.e.
    ports 5000-5100 on WAN interface redirected to a LAN host while original
    destination port is not altered) When different port ranges are configured,
    either 'random' mode should be used, or else all incoming connections are
    mapped onto the first port in the redirect range. (in described example
    WAN:5000-5100 will all be mapped to 192.168.1.5:2000)

    This patch introduces a new mode indicated by flag NF_NAT_RANGE_PROTO_OFFSET
    which uses a base port value to calculate an offset with the destination port
    present in the incoming stream. That offset is then applied as index within the
    redirect port range (index modulo rangewidth to handle range overflow).

    In described example the base port would be 5000. An incoming stream with
    destination port 5004 would result in an offset value 4 which means that the
    NAT'ed stream will be using destination port 2004.

    Other possibilities include deterministic mapping of larger or multiple ranges
    to a smaller range : WAN:5000-5999 -> LAN:5000-5099 (maps WAN port 5*xx to port
    51xx)

    This patch does not change any current behavior. It just adds new NAT proto
    range functionality which must be selected via the specific flag when intended
    to use.

    A patch for iptables (libipt_DNAT.c + libip6t_DNAT.c) will also be proposed
    which makes this functionality immediately available.

    Signed-off-by: Thierry Du Tre <thierry@dtsystems.be>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>