mm: fix possible off-by-one in walk_pte_range()

After the loop in walk_pte_range() pte might point to the first address after
the pmd it walks. The pte_unmap() is then applied to something bad.

Spotted by Roel Kluin and Andreas Schwab.

Signed-off-by: Johannes Weiner <[e-mail]>
Cc: Roel Kluin <[e-mail]>
Cc: Andreas Schwab <[e-mail]>
Acked-by: Matt Mackall <[e-mail]>
Acked-by: Mikael Pettersson <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/pagewalk.c

···
 	int err = 0;
 
 	pte = pte_offset_map(pmd, addr);
-	do {
+	for (;;) {
 		err = walk->pte_entry(pte, addr, addr + PAGE_SIZE, private);
 		if (err)
 		       break;
-	} while (pte++, addr += PAGE_SIZE, addr != end);
+		addr += PAGE_SIZE;
+		if (addr == end)
+			break;
+		pte++;
+	}
 
 	pte_unmap(pte);
 	return err;