net: Prevent infinite while loop in skb_tx_hash()

Drivers call netdev_set_num_tc() and then netdev_set_tc_queue()
to set the queue count and offset for each TC.  So the queue count
and offset for the TCs may be zero for a short period after dev->num_tc
has been set.  If a TX packet is being transmitted at this time in the
code path netdev_pick_tx() -> skb_tx_hash(), skb_tx_hash() may see
nonzero dev->num_tc but zero qcount for the TC.  The while loop that
keeps looping while hash >= qcount will not end.

Fix it by checking the TC's qcount to be nonzero before using it.

Fixes: eadec877 ("net: Add support for subordinate traffic classes to netdev_pick_tx")
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>