x86: Restore proper vector locking during cpu hotplug

Having cpu_online_map change during assign_irq_vector can result
in some really nasty and weird things happening.  The one that
bit me last time was accessing non existent per cpu memory for non
existent cpus.

This locking was removed in a sloppy x86_64 and x86_32 merge patch.

Guys can we please try and avoid subtly breaking x86 when we are
merging files together?

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>