internal/poll: avoid unnecessary memory allocation in Writev

Writev was allocating a new []syscall.Iovec every call, rather than
reusing the cached copy available at *fd.iovec.

Fixes #26663.

Change-Id: I5967b0d82dc671ce0eaf4ec36cc2a0e46eadde02
Reviewed-on: https://go-review.googlesource.com/c/go/+/172419
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/writev.go

@@ -51,7 +51,10 @@ func (fd *FD) Writev(v *[][]byte) (int64, error) {
 		if len(iovecs) == 0 {
 			break
 		}
-		fd.iovecs = &iovecs // cache
+		if fd.iovecs == nil {
+			fd.iovecs = new([]syscall.Iovec)
+		}
+		*fd.iovecs = iovecs // cache
 
 		var wrote uintptr
 		wrote, err = writev(fd.Sysfd, iovecs)