btrfs: atomically insert the new extent in btrfs_split_ordered_extent

Currently there is a small race window in btrfs_split_ordered_extent,
where the reduced old extent can be looked up on the per-inode rbtree
or the per-root list while the newly split out one isn't visible yet.

Fix this by open coding btrfs_alloc_ordered_extent in
btrfs_split_ordered_extent, and holding the tree lock and
root->ordered_extent_lock over the entire tree and extent manipulation.

Note that this introduces new lock ordering because previously
ordered_extent_lock was never held over the tree lock.

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>