net: Clean up explicit ->tx_queue references in link watch.

net: Clean up explicit ->tx_queue references in link watch.

First, we add a qdisc_tx_changing() helper which returns true if the
qdisc attachment is in transition.

Second, we remove an assertion warning which is of limited value and
is hard to express precisely in a multiqueue environment.

Signed-off-by: David S. Miller <davem@davemloft.net>