It shouldn't be necessary to use a barrier on the way into
spin_lock. We'll be focused on a single address until we get
it (exclusively) set, and then we'll do a barrier on the way
out. Also, it does make sense to do a barrier on the way in
to spin_unlock, i.e. ensure what we did in the critical section
is ordered wrt to what we do outside it, before we announce that
we're outside.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>