ipv6: lower "link become ready"'s level message

This following message is printed in the console each time a network
device configured with an IPv6 addresses is ready to be used:

  ADDRCONF(NETDEV_CHANGE): <iface>: link becomes ready

When netns are being extensively used -- e.g. by re-creating netns' with
veth to discuss with each others for testing purposes like mptcp_join.sh
selftest does -- it generates a lot of messages like that: more than 700
when executing mptcp_join.sh with the latest version.

It looks like this message is not that helpful after all: maybe it can
be used as a sign to know if there is something wrong, e.g. if a device
is being regularly reconfigured by accident? But even then, there are
better ways to monitor and diagnose such issues.

When looking at commit 3c21edb ("[IPV6]: Defer IPv6 device
initialization until the link becomes ready.") which introduces this new
message, it seems it had been added to verify that the new feature was
working as expected. It could have then used a lower level than "info"
from the beginning but it was fine like that back then: 17 years ago.

It seems then OK today to simply lower its level, similar to commit
7c62b8d ("net/ipv6: lower the level of "link is not ready" messages")
and as suggested by Mat [1], Stephen and David [2].

Link: https://lore.kernel.org/mptcp/614e76ac-184e-c553-af72-084f792e60b0@kernel.org/T/ [1]
Link: https://lore.kernel.org/netdev/68035bad-b53e-91cb-0e4a-007f27d62b05@tessares.net/T/ [2]
Suggested-by: Mat Martineau <martineau@kernel.org>
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Suggested-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -3633,8 +3633,8 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
 				idev->if_flags |= IF_READY;
 			}
 
-			pr_info("ADDRCONF(NETDEV_CHANGE): %s: link becomes ready\n",
-				dev->name);
+			pr_debug("ADDRCONF(NETDEV_CHANGE): %s: link becomes ready\n",
+				 dev->name);
 
 			run_pending = 1;
 		}