pg_upgrade --link fails: `directory "/..." already in use as a tablespace`
"PG_10_201707211" must be a left over directory from your aborted attempt at running pg_upgrade without the --link flag.  You can delete it, or out of an abundance of caution move it to one of your other partitions.  Renaming it the way you describe wouldn't accomplish much and would be very confusing if you have more problems in the future which require more investigation.