[NET]: Dynamically allocate the per cpu counters for the loopback device.

This patch add support for dynamically allocating the statistics counters
for the loopback device and adds appropriate device methods for allocating
and freeing the loopback device.

This completes support for creating multiple instances of the loopback
device,  in preparation for creating per network namespace instances.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>