bonding: disallow enslaving a bond to itself

Enslaving a bond to itself leads to an endless loop and hangs the kernel.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Tested-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>