sch_sfq: allow big packets and be fair

sch_sfq: allow big packets and be fair

SFQ is currently 'limited' to small packets, because it uses a 15bit
allotment number per flow. Introduce a scale by 8, so that we can handle
full size TSO/GRO packets.

Use appropriate handling to make sure allot is positive before a new
packet is dequeued, so that fairness is respected.

Signed-off-by: Eric Dumazet <[email protected]>
Acked-by: Jarek Poplawski <[email protected]>
Cc: Patrick McHardy <[email protected]>
Signed-off-by: David S. Miller <[email protected]>