RDMA/restrack: Don't treat as an error allocation ID wrapping

xa_alloc_cyclic() call returns positive number if ID allocation
succeeded but wrapped. It is not an error, so normalize the "ret"
variable to zero as marker of not-an-error.

   drivers/infiniband/core/restrack.c:261 rdma_restrack_add()
   warn: 'ret' can be either negative or positive

Fixes: fd47c2f9

 ("RDMA/restrack: Convert internal DB from hash to XArray")
Link: https://lore.kernel.org/r/20201216100753.1127638-1-leon@kernel.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>