Revert "net-loopback: set lo dev initial state to UP"

In commit c9dca822 ("net-loopback: set lo dev initial state to UP"),
linux started automatically bringing up the loopback device of a newly
created namespace. However, an existing user script might reasonably have
the following stanza when creating a new namespace -- and in fact at least
tools/testing/selftests/net/fib_nexthops.sh in Linux's very own testsuite
does:

 # set -e
 # ip netns add foo
 # ip -netns foo addr add 127.0.0.1/8 dev lo
 # ip -netns foo link set lo up
 # set +e

This will now fail, because the kernel reasonably rejects "ip addr add" of
a duplicate address. The described change of behavior therefore constitutes
a breakage. Revert it.

Fixes: c9dca822 ("net-loopback: set lo dev initial state to UP")
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>