ubifs: use swap macro in swap_dirty_idx

Make use of the swap macro and remove unnecessary variable *t*. This
makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/find.c

@@ -750,10 +750,7 @@ static int cmp_dirty_idx(const struct ubifs_lprops **a,
 static void swap_dirty_idx(struct ubifs_lprops **a, struct ubifs_lprops **b,
 			   int size)
 {
-	struct ubifs_lprops *t = *a;
-
-	*a = *b;
-	*b = t;
+	swap(*a, *b);
 }
 
 /**