RDMA/rtrs-clt: Refactor the failure cases in alloc_clt

Make all failure cases go to the common path to avoid duplicate code.
And some issued existed before.

1. clt need to be freed to avoid memory leak.

2. return ERR_PTR(-ENOMEM) if kobject_create_and_add fails, because
   rtrs_clt_open checks the return value of by call "IS_ERR(clt)".

Fixes: 6a98d71d ("RDMA/rtrs: client: main functionality")
Link: https://lore.kernel.org/r/20201217141915.56989-15-jinpu.wang@cloud.ionos.com


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