RDMA/rtrs-srv: Jump to dereg_mr label if allocate iu fails

The rtrs_iu_free is called in rtrs_iu_alloc if memory is limited, so we
don't need to free the same iu again.

Fixes: 9cb83748 ("RDMA/rtrs: server: main functionality")
Link: https://lore.kernel.org/r/20201217141915.56989-7-jinpu.wang@cloud.ionos.com


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