RDMA/srpt: Introduce a reference count in struct srpt_device

This will be used to keep struct srpt_device around as long as either the
RDMA port exists or a LIO target port is associated with the struct
srpt_device.

Link: https://lore.kernel.org/r/20220727193415.1583860-3-bvanassche@acm.org


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>