btrfs: split btrfs_alloc_ordered_extent to allocation and insertion helpers

Split two low-level helpers out of btrfs_alloc_ordered_extent to allocate
and insert the logic extent.  The pure alloc helper will be used to
improve btrfs_split_ordered_extent.

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