syncmap: release m.mu during (*RWMutexMap).Range callbacks

The mainline syncmap.Map has allowed mutations within Range callbacks
since https://golang.org/cl/37342. The reference
implementations used in map_bench_test need to do the same.

Change-Id: Id73d254fa01cc64a1f00eb1903488796e1282423
Reviewed-on: https://go-review.googlesource.com/42956
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

syncmap/map_bench_test.go

@@ -204,12 +204,10 @@ func BenchmarkAdversarialDelete(b *testing.B) {
 				m.Load(i)
 
 				if i%mapSize == 0 {
-					var key int
 					m.Range(func(k, _ interface{}) bool {
-						key = k.(int)
+						m.Delete(k)
 						return false
 					})
-					m.Delete(key)
 					m.Store(i, i)
 				}
 			}