rcu: Add destroy_work_on_stack() to match INIT_WORK_ONSTACK()

The synchronize_rcu_expedited() function has an INIT_WORK_ONSTACK(),
but lacks the corresponding destroy_work_on_stack().  This commit
therefore adds destroy_work_on_stack().

Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>