net: ipc_router: Flush xprt workqueue before removing routing table entry

During SSR removing the routing table entry before flushing the xprt
reader workqueue is allowing to add a duplicate routing table entry
again with removed xprt pointer and causing a xprt access after free.

Flush the xprt reader workqueue and free all pending packets from
the list before removing the routing table entry.

CRs-Fixed: 874846
Change-Id: I2f858252bb5f7a7b6382b42011ad524da3fffe87
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>