f2fs: sync to upstream

https://git.kernel.org/cgit/linux/kernel/git/jaegeuk/f2fs.git/commit/?h=linux-3.4&id=994642cfdbc08f1eda7ff48504f779cbcc9e3067

Change-Id: I5beb230d4a5dc3ef3913f5bfc4c28b39bcb755b1
34 files changed