f2fs: don't track volatile file in dirty inode list

Don't track volatile file in dirty inode list, otherwise with data_flush
option, background thread will entry into endless loop for flushing
journal file's pages.

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