btrfs: factor out a can_finish_ordered_extent helper

Factor out a helper from btrfs_mark_ordered_io_finished that does the
actual per-ordered_extent work to check if we want to schedule an I/O
completion.  This new helper will later be used complete an
ordered_extent without first doing a lookup.

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