RDMA/uverbs: Fix an error flow in ib_uverbs_poll_cq

The new output_written block was wrongly placed before the ret=0, causing
the error code to be lost. uverbs_output_written is not expected to fail,
and even if it does fail it has no significant impact on the userspace
flow.

Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: d6f4a21f

 ("RDMA/uverbs: Mark ioctl responses with UVERBS_ATTR_F_VALID_OUTPUT")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>