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
(cherry picked from commit fa8c9e7)

Co-authored-by: Jakub Kulík <[email protected]>

Author: miss-islington

Modules/posixmodule.c

@@ -9456,14 +9456,13 @@ 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;