Add back vacuum_cleanup_index_scale_factor parameter.

Commit 9f3665f removed the vacuum_cleanup_index_scale_factor storage
parameter.  However, that creates dump/reload hazards when moving across
major versions.

Add back the vacuum_cleanup_index_scale_factor parameter (though not the
GUC of the same name) purely to avoid problems when using tools like
pg_upgrade.  The parameter remains disabled and undocumented.

No backpatch to Postgres 13, since vacuum_cleanup_index_scale_factor was
only disabled by REL_13_STABLE's version of master branch commit
9f3665f in the first place -- the parameter already looks like this on
REL_13_STABLE.

Discussion: https://postgr.es/m/YEm/a3Ko3nKnBuVq@paquier.xyz

Author: petergeoghegan

src/backend/access/common/reloptions.c

@@ -461,6 +461,15 @@ static relopt_real realRelOpts[] =
 		},
 		0, -1.0, DBL_MAX
 	},
+	{
+		{
+			"vacuum_cleanup_index_scale_factor",
+			"Deprecated B-Tree parameter.",
+			RELOPT_KIND_BTREE,
+			ShareUpdateExclusiveLock
+		},
+		-1, 0.0, 1e10
+	},
 	/* list terminator */
 	{{NULL}}
 };

src/backend/access/nbtree/nbtutils.c

@@ -2105,6 +2105,8 @@ btoptions(Datum reloptions, bool validate)
 {
 	static const relopt_parse_elt tab[] = {
 		{"fillfactor", RELOPT_TYPE_INT, offsetof(BTOptions, fillfactor)},
+		{"vacuum_cleanup_index_scale_factor", RELOPT_TYPE_REAL,
+		offsetof(BTOptions, vacuum_cleanup_index_scale_factor)},
 		{"deduplicate_items", RELOPT_TYPE_BOOL,
 		offsetof(BTOptions, deduplicate_items)}
 

src/include/access/nbtree.h

@@ -1067,6 +1067,7 @@ typedef struct BTOptions
 {
 	int32		varlena_header_;	/* varlena header (do not touch directly!) */
 	int			fillfactor;		/* page fill factor in percent (0..100) */
+	float8		vacuum_cleanup_index_scale_factor; /* deprecated */
 	bool		deduplicate_items;	/* Try to deduplicate items? */
 } BTOptions;