net: rmnet_data: consolidate VND free work-queue task on force unassociate

Previously, one new work-queue item was created and scheduled with
schedule_work() for each VND getting unregistered. Since we know the
exact set of VNDs which need to be cleared ahead of time, the VNDs
are added to a list and freed at the same time with a single work-queue
task. This saves us from having to malloc/schedule/free for each
VND and provides a speed up on some low tier hardware.

CRs-Fixed: 738039
Change-Id: I02d4de1308a2aed9d493f6fd58cf0984265facba
Acked-by: Nagarjuna Chaganti <nchagant@qti.qualcomm.com>
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>