sch_sfq: validate silly quantum values

syzbot managed to set up sfq so that q->scaled_quantum was zero,
triggering an infinite loop in sfq_dequeue()

More generally, we must only accept quantum between 1 and 2^18 - 7,
meaning scaled_quantum must be in [1, 0x7FFF] range.

Otherwise, we also could have a loop in sfq_dequeue()
if scaled_quantum happens to be 0x8000, since slot->allot
could indefinitely switch between 0 and 0x8000.

Fixes: eeaeb068

 ("sch_sfq: allow big packets and be fair")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by:  <syzbot+0251e883fe39e7a0cb0a@syzkaller.appspotmail.com>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>