bonding: allow nesting of bonding device

The commit 3c9ef511 ("bonding: avoid adding slave device with
IFF_MASTER flag") fix a crash when add slave device with IFF_MASTER,
but it rejects the scenario of nested bonding device.

As Eric Dumazet described: since there indeed is a usage scenario about
nesting bonding, we should not break it.

So we add a new judgment condition to allow nesting of bonding device.

Fixes: 3c9ef511

 ("bonding: avoid adding slave device with IFF_MASTER flag")
Suggested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Di Zhu <zhudi21@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>