f2fs: atomic: fix to truncate pagecache before on-disk metadata truncation

We should always truncate pagecache while truncating on-disk data.

Fixes: a46bebd5 ("f2fs: synchronize atomic write aborts")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>