RDMA/uverbs: Simplify ib_uverbs_ex_query_device

We truncate the response structure if there is not enough room in the
user buffer so there is no reason to have all the mess with finely managing
response_length. Just fully fill the attrs and truncate on copy.

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