[NET]: Proper comment for loopback initialization order.

Loopback device is special. It should be initialized at the very
beginning.  Initialization order has been changed by
Eric W. Biederman <ebiederm@xmission.com> and this change is non-obvious
and important enough to add proper comment.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>