unix: refactor uv__fs_copyfile() logic

The Unix/macOS uv__fs_copyfile() implementation falls back to
using uv_fs_sendfile(). This commit refactors the error handling
to use the sendfile() req's result field, which is an ssize_t
instead of using the return value, which is an int. The int
value was coming back as a negative number for some large files.

Refs: nodejs/node#30085

src/unix/fs.c

@@ -1076,16 +1076,12 @@ static ssize_t uv__fs_copyfile(uv_fs_t* req) {
   bytes_to_send = src_statsbuf.st_size;
   in_offset = 0;
   while (bytes_to_send != 0) {
-    err = uv_fs_sendfile(NULL,
-                         &fs_req,
-                         dstfd,
-                         srcfd,
-                         in_offset,
-                         bytes_to_send,
-                         NULL);
+    uv_fs_sendfile(NULL, &fs_req, dstfd, srcfd, in_offset, bytes_to_send, NULL);
     uv_fs_req_cleanup(&fs_req);
-    if (err < 0)
+    if (fs_req.result < 0) {
+      err = fs_req.result;
       break;
+    }
     bytes_to_send -= fs_req.result;
     in_offset += fs_req.result;
   }