darwin: make uv_fs_sendfile() respect length param

The darwin sendfile implementation uses the &len parameter as input
and output. The code was sending 0 (not using the value of req->len)
so the behavior wasn't what the caller was expecting.

This makes sure to initialize len with req->len to ensure that the
caller can send portions of a file (not always everything to the
end of the file).

src/unix/fs.c

@@ -436,11 +436,14 @@ static ssize_t uv__fs_sendfile(uv_fs_t* req) {
      * non-blocking mode and not all data could be written. If a non-zero
      * number of bytes have been sent, we don't consider it an error.
      */
-    len = 0;
 
 #if defined(__FreeBSD__)
+    len = 0;
     r = sendfile(in_fd, out_fd, req->off, req->len, NULL, &len, 0);
 #else
+    /* The darwin sendfile takes len as an input for the length to send,
+     * so make sure to initialize it with the caller's value. */
+    len = req->len;
     r = sendfile(in_fd, out_fd, req->off, &len, NULL, 0);
 #endif