RDMA/rtrs: Fix some signedness bugs in error handling

The problem is that "req->sg_cnt" is an unsigned int so if "nr" is
negative, it gets type promoted to a high positive value and the condition
is false.  This patch fixes it by handling negatives separately.

Fixes: 6a98d71d ("RDMA/rtrs: client: main functionality")
Link: https://lore.kernel.org/r/20200519133223.GN2078@kadam


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>