IB/mlx5: Expose correct max_fast_reg_page_list_len

While documentation indicates that the number of translation
entries per memory key is unlimited, in practice, we can
only fit a finite amount of translation entries in a single
registration wqe (which is log_max_klm_list_size).

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>