internal/poll: avoid memory leak in Writev

The chunks that were referenced by fd.iovecs would not be GC.

Change-Id: I7bfcb91a3fef57a4a1861168e9cd3ab55ce1334e
GitHub-Last-Rev: e0b7f68447441fd89ed1a6e8aa37e2084fd863b2
GitHub-Pull-Request: golang/go#32138
Reviewed-on: https://go-review.googlesource.com/c/go/+/178037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/writev.go

@@ -64,6 +64,9 @@ func (fd *FD) Writev(v *[][]byte) (int64, error) {
 		TestHookDidWritev(int(wrote))
 		n += int64(wrote)
 		consume(v, int64(wrote))
+		for i := range iovecs {
+			iovecs[i] = syscall.Iovec{}
+		}
 		if err != nil {
 			if err.(syscall.Errno) == syscall.EAGAIN {
 				if err = fd.pd.waitWrite(fd.isFile); err == nil {