PI offload (protection information) is a feature that each RDMA provider can implement differently. Thus, introduce new device attribute to define the maximal length of the page list for PI fast registration operation. For example, mlx5 driver uses a single internal MR to map both data and protection SGL's, so it's equal to max_fast_reg_page_list_len / 2.
This is a result of using a single MR, you guys know the use cases better than I do at this point, is it acceptable? Because you could have used more than a single MR (although it seems reasonable to me given the overhead of allocating extra MRs for it...)
