timer: Fix up timer-state teardown on CPU offline / online-failure.

timer: Fix up timer-state teardown on CPU offline / online-failure.

The lock-free access to timer->cpu in timer_lock() is problematic, as
the per-cpu data that is then dereferenced could disappear under our
feet. Now that per-cpu data is freed via RCU, we simply add a RCU
read-side critical section to timer_lock(). It is then also
unnecessary to migrate timers on CPU_DYING (we can defer it to the
nicer CPU_DEAD state) and we can also safely migrate timers on
CPU_UP_CANCELED.

Signed-off-by: Keir Fraser <keir@xen.org>