x86: fix assertion in get_page_type()

c/s 22998:e9fab50d7b61 (and immediately following ones) made it
possible that __get_page_type() returns other than -EINVAL, in
particular -EBUSY. Consequently, the assertion in get_page_type()
should check for only the return values we absolutely don't expect to
see there.

This is XSA-37 / CVE-2013-0154.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2318,7 +2318,7 @@ int get_page_type(struct page_info *page
     int rc = __get_page_type(page, type, 0);
     if ( likely(rc == 0) )
         return 1;
-    ASSERT(rc == -EINVAL);
+    ASSERT(rc != -EINTR && rc != -EAGAIN);
     return 0;
 }