ifb: should not use __dev_get_by_index() without locks

At this point (ri_tasklet()), RTNL or dev_base_lock are not held,
we must use dev_get_by_index() instead of __dev_get_by_index()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>