RDMA/umem: Make ib_umem_odp into a sub structure of ib_umem

These two structures are linked together, use the container_of pattern
instead of a double allocation to make the code simpler and easier to
follow.

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