net: sched: Schedule PRIO qdisc when flow control released

The PRIO qdisc supports flow control, such that packet
dequeue can be disabled based on boolean flag 'enable_flow'.
When flow is re-enabled, the latency for new packets
arriving at network driver is high.  To reduce the delay in
scheduling packets, the qdisc will now invoke
__netif_schedule() to expedite dequeue.  This significantly
reduces the latency of packets arriving at network driver.

Change-Id: Ic5fe3faf86f177300d3018b9f60974ba3811641c
CRs-Fixed: 355156
Acked-by: Jimi Shah <jimis@qualcomm.com>
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>
net: sched: export an api to enable/disable flow on sch

Export a function from sch_api.c that will look up
desired qdisc and call it's registered change function
to enable/disable flow.

Change-Id: I5b6dc7a6fd2b09b796c92b3770ba83423d19c864
CRs-Fixed: 355156
Acked-by: Jimi Shah <jimis@qualcomm.com>
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>
(cherry picked from commit b8419fe690053b76658d49565c57ac654faf2eaa)
(cherry picked from commit 3a30e7aa4487f56a74f12c12f11cece6ce1f2100)

Conflicts:
	include/net/pkt_sched.h
	net/sched/sch_api.c