xen: credit2: fix spinlock irq-safety violation

xen: credit2: fix spinlock irq-safety violation

In commit ad4b3e1e9df34 ("xen: credit2: implement
utilization cap") xfree() was being called (for
deallocating the budget replenishment timer, during
domain destruction) inside an IRQ disabled critical
section.

That must not happen, as it uses the mem-pool's lock,
which needs to be taken with IRQ enabled. And, in fact,
we crash (in debug builds):

(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Xen BUG at spinlock.c:47
(XEN) ****************************************

Let's, therefore, kill and deallocate the timer outside of
the critical sections, when IRQs are enabled.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>