RDMA/srpt: Make the code for handling port identities more systematic

Introduce a new data structure for the information about an RDMA port
name. This patch does not change any functionality.

Link: https://lore.kernel.org/r/20190930231707.48259-15-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>