net: return poll.SendFile error from sendFile

We were accidentally ignoring any error returned by poll.SendFile.
Noticed by reading the code. It could only change behavior if the
sendfile system call both wrote some bytes and returned an error.

Change-Id: I0693d6ec0a30f5a86b78d38793899ca29fb9e156
Reviewed-on: https://go-review.googlesource.com/c/164760
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/sendfile_linux.go

@@ -42,8 +42,8 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 		written, werr = poll.SendFile(&c.pfd, int(fd), remain)
 		return true
 	})
-	if werr == nil {
-		werr = err
+	if err == nil {
+		err = werr
 	}
 
 	if lr != nil {

src/net/sendfile_unix_alt.go

@@ -68,8 +68,8 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 		written, werr = poll.SendFile(&c.pfd, int(fd), pos, remain)
 		return true
 	})
-	if werr == nil {
-		werr = err
+	if err == nil {
+		err = werr
 	}
 
 	if lr != nil {