net/core: print message for allmulticast

When the user sets or clears the IFF_ALLMULTI flag in the netdev, there are
no log messages printed to the kernel log to indicate anything happened.
This is inexplicably different from most other dev->flags changes, and
could suprise the user.

Typically this occurs from user-space when a user:
ip link set eth0 allmulticast <on|off>

However, other devices like bridge set allmulticast as well, and many
other flows might trigger entry into allmulticast as well.

The new message uses the standard netdev_info print and looks like:
[  413.246110] ixgbe 0000:17:00.0 eth0: entered allmulticast mode
[  415.977184] ixgbe 0000:17:00.0 eth0: left allmulticast mode
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/dev.c

@@ -8391,6 +8391,8 @@ static int __dev_set_allmulti(struct net_device *dev, int inc, bool notify)
 		}
 	}
 	if (dev->flags ^ old_flags) {
+		netdev_info(dev, "%s allmulticast mode\n",
+			    dev->flags & IFF_ALLMULTI ? "entered" : "left");
 		dev_change_rx_flags(dev, IFF_ALLMULTI);
 		dev_set_rx_mode(dev);
 		if (notify)