RDMA/verbs: Add a DMA iterator to return aligned contiguous memory blocks

This helper iterates over a DMA-mapped SGL and returns contiguous memory
blocks aligned to a HW supported page size.

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