f2fs: backport performance improvements

This commit squashes in the following patches as backports:

- f2fs: skip unnecessary data writes during fsync
  https://github.com/torvalds/linux/commit/6d99ba41a72c1213f2029e6fc66aa7943339b5db

- f2fs: skip unnecessary node writes during fsync
  https://github.com/torvalds/linux/commit/479f40c44ae30e02642ce0391be707a53852d545

- f2fs: readahead multi pages of directory for performance
  https://github.com/torvalds/linux/commit/817202d937e6cca7e60f42e6495aaa51d70d9d7e

Signed-off-by: Blechd0se <alex.christ@hotmail.de>