RDMA/rtrs-clt: Kill rtrs_clt_change_state

It is just a wrapper of rtrs_clt_change_state_get_old, and we can reuse
rtrs_clt_change_state_get_old with add the checking of 'old_state' is
valid or not.

Link: https://lore.kernel.org/r/20201217141915.56989-12-jinpu.wang@cloud.ionos.com


Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>