FROMGIT: f2fs: assign the write hint per stream by default

This reverts commit 930e2607 ("f2fs: remove obsolete whint_mode"), as we
decide to pass write hints to the disk.

Cc: Hyunchul Lee <cheol.lee@lge.com>
Change-Id: I4d76ed3d8233b847f2c8decf8f5e8d7162fd5db4
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 7643f3fe
https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)