RDMA/rxe: Use for_each_sg_page iterator on umem SGL

The driver walks the umem SGL assuming a 1:1 mapping between SGE and
system page. Update to use the for_each_sg_page iterator to get individual
pages contained in the SGEs.  This is a pre-requisite before adding page
combining into SGEs while building the scatter table in IB core.

Additionally, purge umem->page_shift usage in the driver as its only
relevant for ODP MRs. Use system page size and shift instead.

Signed-off-by: Shiraz, Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>