msm_rmnet: add ioctl to enable/disable flow on prio qdisc

This ioctl provides a shorter route to enable/disable flow
on a given qdisc when compared to the traditional NETLINK
based communication. It is known to improve throughput when
data flow is throttled at a higher rate.

Change-Id: I8533887c8a7caffe3b17320dc044f28388ba8b83
CRs-Fixed: 355156
Acked-by: Jimi Shah <jimis@qualcomm.com>
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>