f2fs: multidevice: support direct IO

f2fs: multidevice: support direct IO

Commit 4ced654e00c9 ("f2fs: support multiple devices") missed
to support direct IO for multiple device feature, this patch
adds to support the missing part of multidevice feature.

In addition, for multiple device image, we should be aware of
any issued direct write IO rather than just buffered write IO,
so that fsync and syncfs can issue a preflush command to the
device where direct write IO goes, to persist user data for
posix compliant.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>