x86: remove set_fixmap() warning

set_fixmap()+clear_fixmap() is safe.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/init_64.c

@@ -173,7 +173,7 @@ set_pte_phys(unsigned long vaddr, unsigned long phys, pgprot_t prot)
 	new_pte = pfn_pte(phys >> PAGE_SHIFT, prot);
 
 	pte = pte_offset_kernel(pmd, vaddr);
-	if (!pte_none(*pte) &&
+	if (!pte_none(*pte) && pte_val(new_pte) &&
 	    pte_val(*pte) != (pte_val(new_pte) & __supported_pte_mask))
 		pte_ERROR(*pte);
 	set_pte(pte, new_pte);