RDMA/uverbs: Fill in the response for IB_USER_VERBS_EX_CMD_MODIFY_QP

A response struct was defined, and userspace is providing it (but not
checking it). Fill it in and write it out.

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