[IA64] protect ridblock_owner.

[IA64] protect ridblock_owner.

protect ridblock_owner by spin lock.
deallocate_rid() is called by arch_domain_destroy() which
is called as rcu callback.
On the other hand allocate_rid() is called from domctl hypercall.
So the access to ridblock_owner is racy.
Protect it by spin lock.
So far probably xend serializes creating domains, so it hasn't
been caused issues.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>