UPSTREAM: f2fs: atomic: fix to forbid dio in atomic_file

atomic write can only be used via buffered IO, let's fail direct IO on
atomic_file and return -EOPNOTSUPP.

Change-Id: I6a0eadcc968ba1001a155784fc074adde6eb37f4
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit b2c160f4)