RDMA/rtrs-srv: Set .release function for rtrs srv device during device init

The device .release function was not being set during the device
initialization. This was leading to the below warning, in error cases when
put_srv was called before device_add was called.

Warning:

Device '(null)' does not have a release() function, it is broken and must
be fixed. See Documentation/kobject.txt.

So, set the device .release function during device initialization in the
__alloc_srv() function.

Fixes: baa5b28b

 ("RDMA/rtrs-srv: Replace device_register with device_initialize and device_add")
Link: https://lore.kernel.org/r/20200907102216.104041-1-haris.iqbal@cloud.ionos.com
Signed-off-by: Md Haris Iqbal <haris.iqbal@cloud.ionos.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>