[PKT_SCHED]: Qdisc drop operation is optional

The drop operation is optional and qdiscs must check if childs support it.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/sched/sch_netem.c
Simple merge

diff --cc net/sched/sch_prio.c
Simple merge

diff --cc net/sched/sch_tbf.c
Simple merge