x86: On CPU offline, fix master waiting for slave to be fully dead.

x86: On CPU offline, fix master waiting for slave to be fully dead.

On two back-to-back CPU offline operations, on second offline the
cpu_state var will be CPU_STATE_DEAD from the first offline. Hence
__cpu_die() will incorrectly not wait for the second slave to fully
die and set cpu_state itself.

The fix is to set cpu_state to a new value, CPU_STATE_DYING, earlier
during CPU offline, before __cpu_die() starts to execute.

Original diagnosis and patch by Liu, Jinsong <jinsong.liu@intel.com>

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