net: refactoring in preparation for integrated network poller
Introduce pollDesc struct, to split netFD struct into fd-related
and poller-related parts.

R=golang-dev, bradfitz, iant
CC=golang-dev
https://golang.org/cl/7762044
diff --git a/src/pkg/net/sendfile_linux.go b/src/pkg/net/sendfile_linux.go
index 3357e65..6f1323b 100644
--- a/src/pkg/net/sendfile_linux.go
+++ b/src/pkg/net/sendfile_linux.go
@@ -59,7 +59,7 @@
 			break
 		}
 		if err1 == syscall.EAGAIN {
-			if err1 = c.pollServer.WaitWrite(c); err1 == nil {
+			if err1 = c.pd.WaitWrite(); err1 == nil {
 				continue
 			}
 		}