f2fs: allocate hot_data for atomic write more strictly

If a file not set type as hot, has dirty pages more than
threshold 64 before starting atomic write, may be lose hot
flag.

v1->v2: move set FI_ATOMIC_FILE flag behind flush dirty pages too,
in case of dirty pages before starting atomic use atomic mode to
write back.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>