net: tc_qdisc_flow_control returning qdisc size

Changed the tc_qdisc_flow_control API to return the size of the
qdisc in order to be able to collect data on the size of the
qdisc before doing flow control operations. This is required to effectively
diagnose the state of the queues when debugging flow control.

CRs-Fixed: 657414
Change-Id: I4aff2157410e1170de2d0791757ed2e12830a2db
Acked-by: Sivan Reinstein <sivanr@qti.qualcomm.com>
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>