xen/xsm: fix incorrect handling of XSM hook return

If the XSM hook denied access, the execution incorrectly continued on
after an extra unlock domain.

Reported-by: John McDermott <john.mcdermott@nrl.navy.mil>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>
xen-unstable changeset: 24684:833d80d3fbe7
xen-unstable date: Thu Feb 02 15:23:04 2012 +0000

     1.1 --- a/xen/arch/x86/domctl.c	Mon Feb 13 17:57:47 2012 +0000
     1.2 +++ b/xen/arch/x86/domctl.c	Thu Feb 23 10:30:50 2012 +0000
     1.3 @@ -646,7 +646,7 @@ long arch_do_domctl(
     1.4  
     1.5          ret = xsm_machine_address_size(d, domctl->cmd);
     1.6          if ( ret )
     1.7 -            rcu_unlock_domain(d);
     1.8 +            goto set_machine_address_size_out;
     1.9  
    1.10          ret = -EBUSY;
    1.11          if ( d->tot_pages > 0 )