rpcrdma: Implement generic device removal

Commit e87a911f ("nvme-rdma: use ib_client API to detect device
removal") explains the benefits of handling device removal outside
of the CM event handler.

Sketch in an IB device removal notification mechanism that can be
used by both the client and server side RPC-over-RDMA transport
implementations.

Suggested-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>