bonding: disable netpoll on enslave failure

slave_disable_netpoll() is not called upon enslave failure which would
lead to a memory leak. Call slave_disable_netpoll() after err_detach as
that's the first error path after enabling netpoll on that slave.

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