x86/MSI: fix locking in pci_restore_msi_state()

Right after the loop the lock is being dropped, so all loop exits
should happen with the lock still held.

Reported-by: Kristoffer Egefelt <kristoffer@xxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Kristoffer Egefelt <kristoffer@xxxxxxx>

--- a/xen/arch/x86/msi.c
+++ b/xen/arch/x86/msi.c
@@ -1158,11 +1158,11 @@ int pci_restore_msi_state(struct pci_dev
         for ( i = 0; ; )
         {
             msi_set_mask_bit(desc, entry[i].msi_attrib.masked);
-            spin_unlock_irqrestore(&desc->lock, flags);
 
             if ( !--nr )
                 break;
 
+            spin_unlock_irqrestore(&desc->lock, flags);
             desc = &irq_desc[entry[++i].irq];
             spin_lock_irqsave(&desc->lock, flags);
             if ( desc->msi_desc != entry + i )