RDMA/rtrs-clt: Simplify error message

Two error messages are only different message but have common
code to generate the path string.

Link: https://lore.kernel.org/r/20210406123639.202899-4-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>