vim-patch:8.2.3509: undo file is not synced (#23371)

Problem:    Undo file is not synced. (Sami Farin)
Solution:   Sync the undo file if 'fsync' is set. (Christian Brabandt,
            closes vim/vim#8879, closes vim/vim#8920)

340dd0fbe4

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/undo.c

@@ -1333,6 +1333,10 @@ void u_write_undo(const char *const name, const bool forceit, buf_T *const buf,
   }
 #endif
 
+  if (p_fs && fflush(fp) == 0 && os_fsync(fd) != 0) {
+    write_ok = false;
+  }
+
 write_error:
   fclose(fp);
   if (!write_ok) {