Don't initialize page in {vm,fsm}_extend(), not needed

The read path needs to be able to initialize pages anyway, as relation
extensions are not durable. By avoiding initializing pages, we can, in a
future patch, extend the relation by multiple blocks at once.

Using smgrextend() for {vm,fsm}_extend() is not a good idea in general, as at
least one page of the VM/FSM will be read immediately after, always causing a
cache miss, requiring us to read content we just wrote.

Discussion: https://postgr.es/m/20230301223515.pucbj7nb54n4i4nv@awork3.anarazel.de

src/backend/access/heap/visibilitymap.c

@@ -622,11 +622,9 @@ static void
 vm_extend(Relation rel, BlockNumber vm_nblocks)
 {
 	BlockNumber vm_nblocks_now;
-	PGAlignedBlock pg;
+	PGAlignedBlock pg = {0};
 	SMgrRelation reln;
 
-	PageInit((Page) pg.data, BLCKSZ, 0);
-
 	/*
 	 * We use the relation extension lock to lock out other backends trying to
 	 * extend the visibility map at the same time. It also locks out extension
@@ -662,8 +660,6 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	/* Now extend the file */
 	while (vm_nblocks_now < vm_nblocks)
 	{
-		PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);
-
 		smgrextend(reln, VISIBILITYMAP_FORKNUM, vm_nblocks_now, pg.data, false);
 		vm_nblocks_now++;
 	}

src/backend/storage/freespace/freespace.c

@@ -608,10 +608,9 @@ static void
 fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 {
 	BlockNumber fsm_nblocks_now;
-	PGAlignedBlock pg;
+	PGAlignedBlock pg = {0};
 	SMgrRelation reln;
 
-	PageInit((Page) pg.data, BLCKSZ, 0);
 
 	/*
 	 * We use the relation extension lock to lock out other backends trying to
@@ -649,8 +648,6 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 	/* Extend as needed. */
 	while (fsm_nblocks_now < fsm_nblocks)
 	{
-		PageSetChecksumInplace((Page) pg.data, fsm_nblocks_now);
-
 		smgrextend(reln, FSM_FORKNUM, fsm_nblocks_now,
 				   pg.data, false);
 		fsm_nblocks_now++;