RDMA/uverbs: Use uverbs_request() for request copying

This function properly zero-extends, and zero-checks if the user
buffer is not the same size as the kernel command struct.

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