gofix -r mapdelete

R=golang-dev, r, adg, r, cw
CC=golang-dev
https://golang.org/cl/5266045
diff --git a/src/pkg/syscall/syscall_unix.go b/src/pkg/syscall/syscall_unix.go
index 1590b6d..70ea3bd 100644
--- a/src/pkg/syscall/syscall_unix.go
+++ b/src/pkg/syscall/syscall_unix.go
@@ -87,6 +87,6 @@
 	if errno := m.munmap(uintptr(unsafe.Pointer(&b[0])), uintptr(len(b))); errno != 0 {
 		return errno
 	}
-	m.active[p] = nil, false
+	delete(m.active, p)
 	return 0
 }