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: NPetr Machata <petrm@nvidia.com>
Reviewed-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/loopback.c

@@ -219,12 +219,6 @@ static __net_init int loopback_net_init(struct net *net)
 
 	BUG_ON(dev->ifindex != LOOPBACK_IFINDEX);
 	net->loopback_dev = dev;
-
-	/* bring loopback device UP */
-	rtnl_lock();
-	dev_open(dev, NULL);
-	rtnl_unlock();
-
 	return 0;
 
 out_free_netdev: