Fix order of arguments to SubTransSetParent().

ProcessTwoPhaseBuffer (formerly StandbyRecoverPreparedTransactions)
mixed up the parent and child XIDs when calling SubTransSetParent to
record the transactions' relationship in pg_subtrans.

Remarkably, analysis by Simon Riggs suggests that this doesn't lead to
visible problems (at least, not in non-Assert builds).  That might
explain why we'd not noticed it before.  Nonetheless, it's surely wrong.

This code was born broken, so back-patch to all supported branches.

Discussion: https://postgr.es/m/20110.1492905318@sss.pgh.pa.us

src/backend/access/transam/twophase.c

@@ -2142,7 +2142,7 @@ ProcessTwoPhaseBuffer(TransactionId xid,
 		}
 
 		if (setParent)
-			SubTransSetParent(xid, subxid, overwriteOK);
+			SubTransSetParent(subxid, xid, overwriteOK);
 	}
 
 	return buf;