IB/mlx5: Fix compile issue when ODP disabled

When CONFIG_INFINIBAND_ON_DEMAND_PAGING is not enabled, we were getting
build failures for defined but not used code.  Fix that.

Fixes: 813e90b1 ("IB/mlx5: Add advise_mr() support")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>