In bch2_extent_update(), we have to update the inode if i_size is
changing (the file is being extend) or if i_sectors is changing, but we
want to avoid touching the inode if it's not necessary.

Change sum_sector_overwrites() to also check if there's already data
above where we're writing to - this means we're definitely not extending
the file.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>