afs: Fix the setting of the server responding flag

In the Linux kernel, the following vulnerability has been resolved:

afs: Fix the setting of the server responding flag

In afs_wait_for_operation(), we set transcribe the call responded flag to the server record that we used after doing the fileserver iteration loop - but it's possible to exit the loop having had a response from the server that we've discarded (e.g. it returned an abort or we started receiving data, but the call didn't complete).

This means that op->server might be NULL, but we don't check that before attempting to set the server flag.