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>