RDMA/uverbs: Fix a NULL vs IS_ERR() bug

The uapi_get_object() function returns error pointers, it never returns
NULL.

Fixes: 149d3845 ("RDMA/uverbs: Add a method to introspect handles in a context")
Link: https://lore.kernel.org/r/YJ6Got+U7lz+3n9a@mwanda


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>