btrfs: move transaction abort to the error site in btrfs_create_free_space_tree()

The recommended pattern for transaction abort after error is to place it
right after the error is handled. That way it's easier to locate where
it failed and help debugging.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>