Get heap page max offset with buffer lock held.

On further reflection it seems better to call PageGetMaxOffsetNumber()
after acquiring a buffer lock on the page.  This shouldn't really
matter, but doing it this way is cleaner.

Follow-up to commit 42288174.

Backpatch: 12-, just like commit 42288174

src/backend/access/heap/heapam.c

@@ -7042,10 +7042,10 @@ heap_compute_xid_horizon_for_tuples(Relation rel,
 			xid_horizon_prefetch_buffer(rel, &prefetch_state, 1);
 #endif
 
+			LockBuffer(buf, BUFFER_LOCK_SHARE);
+
 			page = BufferGetPage(buf);
 			maxoff = PageGetMaxOffsetNumber(page);
-
-			LockBuffer(buf, BUFFER_LOCK_SHARE);
 		}
 
 		/*