rcu_lock(current->domain) does not need to disable preemption.

rcu_lock(current->domain) does not need to disable preemption.

If the guest sleeps in hypervisor context, it should not be destroyed
until execution reaches a safe point (i.e., guest context). This is
not implemented yet. :-) But the next patch will rely on it, to allow
an HVM guest to execute hypercalls that indirectly invoke __hvm_copy()
within an rcu_lock_current_domain() region.

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