Revert "mm: make swapin readahead skip over holes Ever since abandoning the virtual scan of processes, for scalability reasons, swap space has been a little more fragmented than before. This can lead to the situation where a large memory user is killed, swap space ends up full of "holes" and swapin readahead is totally ineffective."

This reverts commit 86dae1e796853ce249b607e13c2488bd3a7ea63e.