xsplice: Unmask (aka reinstall NMI handler) if we need to abort.

If we have to abort in xsplice_spin() we end following
the goto abort. But unfortunataly we neglected to unmask.
This patch fixes that.

Reported-by: Martin Pohlack <mpohlack@amazon.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

xen/common/xsplice.c

@@ -1262,9 +1262,10 @@ void check_for_xsplice_work(void)
             arch_xsplice_post_action();
             local_irq_restore(flags);
         }
-        arch_xsplice_unmask();
 
  abort:
+        arch_xsplice_unmask();
+
         per_cpu(work_to_do, cpu) = 0;
         xsplice_work.do_work = 0;