net_sched: gred: eliminate redundant DP prio comparisons

Each pair of DPs only needs to be compared once when searching for
a non-unique prio value.

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>