RDMA/core: Fix error code in stat_get_doit_qp()

We need to set the error codes on these paths.  Currently the only
possible error code is -EMSGSIZE so that's what the patch uses.

Fixes: 83c2c1fc ("RDMA/nldev: Allow get counter mode through RDMA netlink")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20190809101311.GA17867@mwanda


Signed-off-by: Doug Ledford <dledford@redhat.com>