The Linux kernel is inconsistent about how devconf is configured for new
network namespaces between ipv4 and ipv6. The behavior can also be
controlled via net.core.devconf_inherit_init_net in Linux 5.1+ so make
sure to enable forwarding on all and default for both ipv6 and ipv4.

This issue first came up testing on a yocto kernel that had this patch:
 ipv4: net namespace does not inherit network configurations

[0] https://www.kernel.org/doc/html/latest/admin-guide/sysctl/net.html#devconf-inherit-init-net
[1] https://lkml.org/lkml/2014/7/29/119Signed-off-by: Brennan Ashton <brennana@jfrog.com>