RDMA/ocrdma: Remove fbo from MR

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.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>