Revert "mm: Added filesystem dynamic read-ahead (Credits to Chad Gooldman)"

This reverts commit 5f0d4393b2490f1d1e47bf34e6b01157d40303cc.
Revert "tmpfs: add support for read_iter and write_iter"

This reverts commit 769493f46ea2a7ab6c26a54c4506195e48d6c888.