net: expose the master link to sysfs, and remove it from bond

net: expose the master link to sysfs, and remove it from bond

Currently, we can have only one master upper neighbour, so it would be
useful to create a symlink to it in the sysfs device directory, the way
that bonding now does it, for every device. Lower devices from
bridge/team/etc will automagically get it, so we could rely on it.

Also, remove the same functionality from bonding.

CC: Jay Vosburgh <[email protected]>
CC: Andy Gospodarek <[email protected]>
CC: "David S. Miller" <[email protected]>
CC: Eric Dumazet <[email protected]>
CC: Jiri Pirko <[email protected]>
CC: Alexander Duyck <[email protected]>
Signed-off-by: Veaceslav Falico <[email protected]>
Signed-off-by: David S. Miller <[email protected]>