RDMA/mlx5: Change mlx5_ib_populate_pas() to use rdma_for_each_block()

This routine converts the umem SGL into a list of fixed pages for DMA,
which is exactly what rdma_umem_for_each_dma_block() is for, use the
common code directly.

Link: https://lore.kernel.org/r/20201026132314.1336717-2-leon@kernel.org


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