RDMA/vmw_pvrdma: Avoid use after free due to QP/CQ/SRQ destroy

The use of wait queues in vmw_pvrdma for handling concurrent
access to a resource leaves a race condition which can cause a use
after free bug.

Fix this by using the pattern from other drivers, complete() protected by
dec_and_test to ensure complete() is called only once.

Fixes: 29c8d9eb

 ("IB: Add vmw_pvrdma driver")
Signed-off-by: Bryan Tan <bryantan@vmware.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>