RDMA/rdmavt: Adapt to handle non-uniform sizes on umem SGEs

rdmavt expects a uniform size on all umem SGEs which is currently at
PAGE_SIZE.

Adapt to a umem API change which could return non-uniform sized SGEs due
to combining contiguous PAGE_SIZE regions into an SGE. Use
for_each_sg_page variant to unfold the larger SGEs into a list of
PAGE_SIZE elements.

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

Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Shiraz, Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>