mtd: nand: atmel: fix buffer overflow in atmel_pmecc_user

When calculating the size needed by struct atmel_pmecc_user *user,
the dmu and delta buffer sizes were forgotten.
This lead to a memory corruption (especially with a large ecc_strength).

Link: http://lkml.kernel.org/r/1506503157.3016.5.camel@gmail.com
Fixes: f88fc122

 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Cc: stable@vger.kernel.org
Reported-by: Richard Genoud <richard.genoud@gmail.com>
Pointed-at-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>