net_sched: sch_sfq: fix allot handling

net_sched: sch_sfq: fix allot handling

When deploying SFQ/IFB here at work, I found the allot management was
pretty wrong in sfq, even changing allot from short to int...

We should init allot for each new flow, not using a previous value found
in slot.

Before patch, I saw bursts of several packets per flow, apparently
denying the default "quantum 1514" limit I had on my SFQ class.

class sfq 11:1 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 7p requeues 0
 allot 11546

class sfq 11:46 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 1p requeues 0
 allot -23873

class sfq 11:78 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 5p requeues 0
 allot 11393

After patch, better fairness among each flow, allot limit being
respected, allot is positive :

class sfq 11:e parent 11:
 (dropped 0, overlimits 0 requeues 86)
 backlog 0b 3p requeues 86
 allot 596

class sfq 11:94 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 3p requeues 0
 allot 1468

class sfq 11:a4 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 4p requeues 0
 allot 650

class sfq 11:bb parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 3p requeues 0
 allot 596

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>