btrfs: remove code duplication in ordered extent finishing

Remove the duplicated transaction joining, block reserve setting and raid
extent inserting in btrfs_finish_ordered_extent().

While at it, also abort the transaction in case inserting a RAID
stripe-tree entry fails.

Suggested-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>