net-loopback: set lo dev initial state to UP

Traditionally loopback devices come up with initial state as DOWN for
any new network-namespace. This would mean that anyone needing this
device would have to bring this UP by issuing something like 'ip link
set lo up'. This can be avoided if the initial state is set as UP.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: Jian Yang <jianyang@google.com>
Link: https://lore.kernel.org/r/20210201233445.2044327-1-jianyang.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>