This is always the same value as IOVA masked by the page size, just use
that clearer calculation directly.

It is unclear of ocrdma hardware can actually support a true fbo, if so it
could use a different algorithm to compute the best page size.

Link: https://lore.kernel.org/r/17-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.comSigned-off-by: Jason Gunthorpe <jgg@nvidia.com>