routing virtualization v2

Hi, Dave!

This set adds namespace support for routing tables & rules manipulation
in the different namespaces. So, one could create a namespace and
setup IPv4 routing there how he wants.

After this patch user will have the ability to configure and
observe its own isolated set of routing rules/tables, but they
all will be unused. I.e. routing decisions inside the network
stack are still made based on the init_net's rules. The reason for doing
so is to have something self-consistent and not too huge :)

The sequence is the following:
- viritualize generic FIB rules operations
- change IPv4 FIB initialization sequence
- virtualize FIB tables access

After this, the 'ip' utility and the /proc interface will start
working correctly inside a namespace, while the 'route' utility
will not, because IP sockets currently cannot be created in non-init
namespace.

Changes from v1:
- fixed spacing in the fib_rules.h
- improved patch description by the suggestion of Daniel Lezcano