runtime: avoid unnecessary map iteration write barrier

Update #14921

Change-Id: I5c5816d0193757bf7465b1e09c27ca06897df4bf
Reviewed-on: https://go-review.googlesource.com/21814
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/hashmap.go

@@ -790,7 +790,9 @@ next:
 				}
 			}
 			it.bucket = bucket
-			it.bptr = b
+			if it.bptr != b { // avoid unnecessary write barrier; see issue 14921
+				it.bptr = b
+			}
 			it.i = i + 1
 			it.checkBucket = checkBucket
 			return