[ARM] 3030/2: fix permission check in the obscur cmpxchg syscall

Patch from Nicolas Pitre

Quoting RMK:

|pte_write() just says that the page _may_ be writable. It doesn't say
|that the MMU is programmed to allow writes. If pte_dirty() doesn't
|return true, that means that the page is _not_ writable from userspace.
|If you write to it from kernel mode (without using put_user) you'll
|bypass the MMU read-only protection and may end up writing to a page
|owned by two separate processes.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/traps.c

@@ -506,7 +506,7 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 		if (!pmd_present(*pmd))
 			goto bad_access;
 		pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
-		if (!pte_present(*pte) || !pte_write(*pte)) {
+		if (!pte_present(*pte) || !pte_dirty(*pte)) {
 			pte_unmap_unlock(pte, ptl);
 			goto bad_access;
 		}