RDMA/srpt: Rework the approach for closing an RDMA channel

Instead of relying on a waitqueue, report when the identity of an RDMA
channel can be reused through a completion.

Link: https://lore.kernel.org/r/20190930231707.48259-13-bvanassche@acm.org


Cc: Honggang LI <honli@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>