Checkpoint and Restart of INET routing information

This set extends the existing network socket, device, and namespace support
in the checkpoint tree to cover routing information.  It does so by making
heavy use of RTNETLINK to dump and insert routes much like userspace would.
Because the task doing the checkpointing or restarting needs to examine
or setup resources for tasks in network namespaces other than its own, an
additional kernel socket setup call is added.  It provides us the ability
to talk to RTNETLINK in a foreign netns.

The support added in this set allows me to configure various inet4 and inet6
routes in a container and have them saved and restored successfully during
a checkpoint/restart process.