RDMA/ucma: Remove closing and the close_wq

Use cancel_work_sync() to ensure that the wq is not running and simply
assign NULL to ctx->cm_id to indicate if the work ran or not. Delete the
close_wq since flush_workqueue() is no longer needed.

Link: https://lore.kernel.org/r/20200818120526.702120-15-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>