f2fs: remove __add_sum_entry

This function just assigns a summary entry.  This can be done entirely
typesafe with an open code struct assignment that relies on array
indexing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>