RDMA/uverbs: Get rid of the 'callback' scheme in the compat path

There is no reason for this. For response processing we simply need to
copy, truncate, and zero fill the response into whatever output buffer
was provided. Add a function uverbs_response() that does this
consistently.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>