RDMA/rtrs: New function converting rtrs_addr to string

RDMA/rtrs: New function converting rtrs_addr to string

There is common code converting addresses of source machine and
destination machine to a string.  We already have a struct rtrs_addr to
store two addresses.  This patch introduces a new function that converts
two addresses into one string with struct rtrs_addr.

Link: https://lore.kernel.org/r/20210325153308.1214057-14-gi-oh.kim@ionos.com
Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>