RDMA/rtrs-srv: Replace device_register with device_initialize and device_add

There are error cases when we will call free_srv before device kobject is
initialized; in such cases calling put_device generates the following
warning:

 kobject: '(null)' (000000009f5445ed): is not initialized, yet
 kobject_put() is being called.

So call device_initialize() only once when the server is allocated. If we
end up calling put_srv() and subsequently free_srv(), our call to
put_device() would result in deletion of the obj. Call device_add() later
when we actually have a connection. Correspondingly, call device_del()
instead of device_unregister() when srv->dev_ref falls to 0.

Fixes: 9cb83748

 ("RDMA/rtrs: server: main functionality")
Link: https://lore.kernel.org/r/20200811092722.2450-1-haris.iqbal@cloud.ionos.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Md Haris Iqbal <haris.iqbal@cloud.ionos.com>
Reviewed-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>