Limit depth of forced recursion for CLOBBER_CACHE_RECURSIVELY.

It's somewhat surprising that we got away with this before.  (Actually,
since nobody tests this routinely AFAIK, it might've been broken for
awhile.  But it's definitely broken in the wake of commit f868a814.)
It seems sufficient to limit the forced recursion to a small number
of levels.

Back-patch to all supported branches, like the preceding patch.

Discussion: https://postgr.es/m/12259.1532117714@sss.pgh.pa.us

src/backend/utils/cache/inval.c

@@ -708,7 +708,17 @@ AcceptInvalidationMessages(void)
 		}
 	}
 #elif defined(CLOBBER_CACHE_RECURSIVELY)
-	InvalidateSystemCaches();
+	{
+		static int	recursion_depth = 0;
+
+		/* Maximum depth is arbitrary depending on your threshold of pain */
+		if (recursion_depth < 3)
+		{
+			recursion_depth++;
+			InvalidateSystemCaches();
+			recursion_depth--;
+		}
+	}
 #endif
 }