RDMA/uverbs: Use the iterator for ib_uverbs_unmarshall_recv()

This has a very complicated memory layout, with two flex arrays. Use
the iterator API to make reading it clearer.

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