net: core: Support UID-based routing.

This contains the following commits:

1. 0b16771 net: ipv6: Add the UID to the route cache.
    This is so that userspace can do per-UID route lookups.

Signed-off-by: paul-xxx <thepaulxxx@gmail.com>
Handle 'sk' being NULL in UID-based routing.

Signed-off-by: paul-xxx <thepaulxxx@gmail.com>