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 <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: "David S. Miller" <davem@davemloft.net>
CC: Eric Dumazet <edumazet@google.com>
CC: Jiri Pirko <jiri@resnulli.us>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>