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>