etf: Drop all expired packets

Currently on dequeue() ETF only drops the first expired packet, which
causes a problem if the next packet is already expired. When this
happens, the watchdog will be configured with a time in the past, fire
straight way and the packet will finally be dropped once the dequeue()
function of the qdisc is called again.

We can save quite a few cycles and improve the overall behavior of the
qdisc if we drop all expired packets if the next packet is expired.
This should allow ETF to recover faster from bad situations. But
packet drops are still a very serious warning that the requirements
imposed on the system aren't reasonable.

This was inspired by how the implementation of hrtimers use the
rb_tree inside the kernel.

Signed-off-by: Jesus Sanchez-Palencia <jesus.s.palencia@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>