datapath: call only into reachable nf-nat code

datapath: call only into reachable nf-nat code

Upstream commit:
    commit 99b7248e2ad57ca93ada10c6598affb267ffc99a
    Author: Arnd Bergmann <arnd@arndb.de>
    Date:   Fri Mar 18 14:33:45 2016 +0100

    openvswitch: call only into reachable nf-nat code

    The openvswitch code has gained support for calling into the
    nf-nat-ipv4/ipv6 modules, however those can be loadable modules
    in a configuration in which openvswitch is built-in, leading
    to link errors:

    net/built-in.o: In function `__ovs_ct_lookup':
    :(.text+0x2cc2c8): undefined reference to `nf_nat_icmp_reply_translation'
    :(.text+0x2cc66c): undefined reference to `nf_nat_icmpv6_reply_translation'

    The dependency on (!NF_NAT || NF_NAT) prevents similar issues,
    but NF_NAT is set to 'y' if any of the symbols selecting
    it are built-in, but the link error happens when any of them
    are modular.

    A second issue is that even if CONFIG_NF_NAT_IPV6 is built-in,
    CONFIG_NF_NAT_IPV4 might be completely disabled. This is unlikely
    to be useful in practice, but the driver currently only handles
    IPv6 being optional.

    This patch improves the Kconfig dependency so that openvswitch
    cannot be built-in if either of the two other symbols are set
    to 'm', and it replaces the incorrect #ifdef in ovs_ct_nat_execute()
    with two "if (IS_ENABLED())" checks that should catch all corner
    cases also make the code more readable.

    The same #ifdef exists ovs_ct_nat_to_attr(), where it does not
    cause a link error, but for consistency I'm changing it the same
    way.

    Signed-off-by: Arnd Bergmann <arnd@arndb.de>
    Fixes: 05752523e565 ("openvswitch: Interface with NAT.")
    Acked-by: Joe Stringer <joe@ovn.org>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Fixes: c5f6c06b58d6 ("datapath: Interface with NAT.")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>