RDMA/odp: Make it clearer when a umem is an implicit ODP umem

RDMA/odp: Make it clearer when a umem is an implicit ODP umem

Implicit ODP umems are special, they don't have any page lists, they don't
exist in the interval tree and they are never DMA mapped.

Instead of trying to guess this based on a zero length use an explicit
flag.

Further, do not allow non-implicit umems to be 0 size.

Link: https://lore.kernel.org/r/20190819111710.18440-4-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>