xen/timers: Fix memory leak with cpu unplug/plug

xen/timers: Fix memory leak with cpu unplug/plug

timer_softirq_action() realloc's itself a larger timer heap whenever
necessary, which includes bootstrapping from the empty dummy_heap.  Nothing
ever freed this allocation.

CPU plug and unplug has the side effect of zeroing the percpu data area, which
clears ts->heap.  This in turn causes new timers to be put on the list rather
than the heap, and for timer_softirq_action() to bootstrap itself again.

This in practice leaks ts->heap every time a CPU is unplugged and replugged.

Implement free_percpu_timers() which includes freeing ts->heap when
appropriate, and update the notifier callback with the recent cpu parking
logic and free-avoidance across suspend.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>