net-sysfs: Add rtnl locking for getting Tx queue traffic class

In order to access the suboordinate dev for a device we should be holding
the rtnl_lock when outside of the transmit path. The existing code was not
doing that for the sysfs dump function and as a result we were open to a
possible race.

To resolve that take the rtnl lock prior to accessing the sb_dev field of
the Tx queue and release it after we have retrieved the tc for the queue.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>