Volatile file's data will be updated oftenly, so it'd better to place
its data into hot data segment.

In addition, for atomic file, we change to check FI_ATOMIC_FILE instead
of FI_HOT_DATA to make code readability better.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>