btrfs: handle completed ordered extents in btrfs_split_ordered_extent

To delay splitting ordered_extents to I/O completion time we need to be
able to handle fully completed ordered extents in
btrfs_split_ordered_extent.  Besides a bit of accounting this primarily
involved moving over the csums to the split bio for the range that it
covers, which is simple enough because we always have one
btrfs_ordered_sum per bio.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>