x86: properly fail mmuext ops when get_page_from_gfn() fails

I noticed this inconsistency while analyzing the code for XSA-32.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2776,7 +2776,7 @@ long do_mmuext_op(
             page = get_page_from_gfn(pg_owner, op.arg1.mfn, NULL, P2M_ALLOC);
             if ( unlikely(!page) )
             {
-                rc = -EINVAL;
+                okay = 0;
                 break;
             }
 
@@ -2836,6 +2836,7 @@ long do_mmuext_op(
             page = get_page_from_gfn(pg_owner, op.arg1.mfn, NULL, P2M_ALLOC);
             if ( unlikely(!page) )
             {
+                okay = 0;
                 MEM_LOG("Mfn %lx bad domain", op.arg1.mfn);
                 break;
             }