We're calling mmdrop() under spin_lock_irq(&rq->lock).  But mmdrop
calls vfree(), which calls smp_call_function().

It is not legal to call smp_call_function() with irq's off.  Because
another CPU may be running smp_call_function() against _this_ CPU, which
deadlocks.

So the patch arranges for mmdrop() to not be called under
spin_lock_irq(&rq->lock).