bpo-41687: Fix error handling in Solaris sendfile implementation (GH-22128)

I just realized that my recent PR with sendfile on Solaris ([PR 22040](#22040)) has broken error handling. Sorry for that, this simple followup fixes that. Automerge-Triggered-By: @1st1

Author: kulikjak

Modules/posixmodule.c

@@ -9521,14 +9521,13 @@ os_sendfile_impl(PyObject *module, int out_fd, int in_fd, PyObject *offobj,
 #if defined(__sun) && defined(__SVR4)
  // On Solaris, sendfile raises EINVAL rather than returning 0
  // when the offset is equal or bigger than the in_fd size.
- int res;
  struct stat st;
 
  do {
  Py_BEGIN_ALLOW_THREADS
- res = fstat(in_fd, &st);
+ ret = fstat(in_fd, &st);
  Py_END_ALLOW_THREADS
- } while (res != 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
+ } while (ret != 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
  if (ret < 0)
  return (!async_err) ? posix_error() : NULL;