RDMA/uverbs: Use uverbs_response() for remaining response copying

This function properly truncates and zero-fills the response which is the
standard used by the ioctl uAPI when working with user data.
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>