RDMA/rxe: Fix unsafe drain work queue code

In the Linux kernel, the following vulnerability has been resolved:

RDMA/rxe: Fix unsafe drain work queue code

If create_qp does not fully succeed it is possible for qp cleanup

code to attempt to drain the send or recv work queues before the

queues have been created causing a seg fault. This patch checks

to see if the queues exist before attempting to drain them.