selftests: bonding: add tests for ether type changes

Add new network selftests for the bonding device which exercise the ether
type changing call paths. They also test for the recent syzbot bug[1] which
causes a warning and results in wrong device flags (IFF_SLAVE missing).
The test adds three bond devices and a nlmon device, enslaves one of the
bond devices to the other and then uses the nlmon device for successful
and unsuccesful enslaves both of which change the bond ether type. Thus
we can test for both MASTER and SLAVE flags at the same time.

If the flags are properly restored we get:
TEST: Change ether type of an enslaved bond device with unsuccessful enslave   [ OK ]
TEST: Change ether type of an enslaved bond device with successful enslave   [ OK ]

[1] https://syzkaller.appspot.com/bug?id=391c7b1f6522182899efba27d891f1743e8eb3ef



Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Acked-by: Jonathan Toppins <jtoppins@redhat.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>