Fix pg_atomic_u64 initialization.

As Andres pointed out, pg_atomic_init_u64 must be used to initialize an
atomic variable, before it can be accessed with the actual atomic ops.
Trying to use pg_atomic_write_u64 on an uninitialized variable leads to a
failure with the fallback implementation that uses a spinlock.

Discussion: https://www.postgresql.org/message-id/20170816191346.d3ke5tpshhco4bnd%40alap3.anarazel.de

src/backend/access/heap/heapam.c

@@ -1638,7 +1638,7 @@ heap_parallelscan_initialize(ParallelHeapScanDesc target, Relation relation,
 		target->phs_nblocks > NBuffers / 4;
 	SpinLockInit(&target->phs_mutex);
 	target->phs_startblock = InvalidBlockNumber;
-	pg_atomic_write_u64(&target->phs_nallocated, 0);
+	pg_atomic_init_u64(&target->phs_nallocated, 0);
 	SerializeSnapshot(snapshot, target->phs_snapshot_data);
 }