x86/hpet: eliminate cpumask_lock

x86/hpet: eliminate cpumask_lock

According to the (now getting removed) comment in struct
hpet_event_channel, this was to prevent accessing a CPU's
timer_deadline after it got cleared from cpumask. This can be done
without a lock altogether - hpet_broadcast_exit() can simply clear
the bit, and handle_hpet_broadcast() can read timer_deadline before
looking at the mask a second time (the cpumask bit was already
found set by the surrounding loop).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Gang Wei <gang.wei@intel.com>