Revert "mm: fix calculation of dirtyable memory"

This reverts commit d9d85280c0ea5e9a8a5f6de1fa657fc0cec052b7.
Revert "writeback: remove nr_pages_dirtied arg from balance_dirty_pages_ratel…"

This reverts commit 8487fbe06dde1e96028ffec8e42a290d3a53e238.
Revert "WRITEBACK: increase bdi_min_ratio to 5 in light of the latest writeback commit (franciscofranco)"

This reverts commit 2f76c51576a2e8bc3b94bbe7a2a59f5d16c200e3.
Revert "writeback: fix writeback cache thrashing"

This reverts commit c994a4c738d7868bcef38ff3acf2a8f797b8953b.