bonding: IFF_BONDING is not stripped on enslave failure

While enslaving a new device and after IFF_BONDING flag is set, in case
of failure it is not stripped from the device's priv_flags while
cleaning up, which could lead to other problems.
Cleaning at err_close because the flag is set after dev_open().

v2: no change

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>