IB/mlx5: Modify to work with arbitrary page size

When dealing with umem objects, the driver assumed host page sizes
defined by PAGE_SHIFT.  Modify the code to use arbitrary page shift
provided by umem->page_shift to support different page sizes.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>