[LINUX] Fix clear_fixmap().

On i386-PAE, clear_fixmap() results in ill use of set_pte(). In all
contexts, p2m translations shouldn't occur here. Note that this is not
really an issue on native linux, as there is
(a) no pfn-to-mfn translation and
(b) __set_fixmap() takes an unsigned long physical address rather than
    a paddr_t, which makes it so that bits 32 and up of the physical
    address are always zero, permitting either order store when
    clearing the entry.

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

linux-2.6-xen-sparse/arch/i386/mm/pgtable-xen.c

@@ -102,8 +102,11 @@ static void set_pte_pfn(unsigned long vaddr, unsigned long pfn, pgprot_t flags)
 		return;
 	}
 	pte = pte_offset_kernel(pmd, vaddr);
-	/* <pfn,flags> stored as-is, to permit clearing entries */
-	set_pte(pte, pfn_pte(pfn, flags));
+	if (pgprot_val(flags))
+		/* <pfn,flags> stored as-is, to permit clearing entries */
+		set_pte(pte, pfn_pte(pfn, flags));
+	else
+		pte_clear(&init_mm, vaddr, pte);
 
 	/*
 	 * It's enough to flush this one mapping.
@@ -140,8 +143,11 @@ static void set_pte_pfn_ma(unsigned long vaddr, unsigned long pfn,
 		return;
 	}
 	pte = pte_offset_kernel(pmd, vaddr);
-	/* <pfn,flags> stored as-is, to permit clearing entries */
-	set_pte(pte, pfn_pte_ma(pfn, flags));
+	if (pgprot_val(flags))
+		/* <pfn,flags> stored as-is, to permit clearing entries */
+		set_pte(pte, pfn_pte_ma(pfn, flags));
+	else
+		pte_clear(&init_mm, vaddr, pte);
 
 	/*
 	 * It's enough to flush this one mapping.

linux-2.6-xen-sparse/arch/x86_64/mm/init-xen.c

@@ -260,7 +260,10 @@ static void set_pte_phys(unsigned long vaddr,
 			return;
 		}
 	}
-	new_pte = pfn_pte(phys >> PAGE_SHIFT, prot);
+	if (pgprot_val(prot))
+		new_pte = pfn_pte(phys >> PAGE_SHIFT, prot);
+	else
+		new_pte = __pte(0);
 
 	pte = pte_offset_kernel(pmd, vaddr);
 	if (!pte_none(*pte) &&