RDMA/mlx5: Use ib_umem_find_best_pgsz() for mkc's

Now that all the PAS arrays or UMR XLT's for mkcs are filled using
rdma_for_each_block() we can use the common ib_umem_find_best_pgsz()
algorithm.

Link: https://lore.kernel.org/r/20201026132314.1336717-6-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>