[UBI] drivers/mtd/ubi/scan.c: kmalloc + memset conversion to kzalloc

To be able to convert kmalloc + memset(..., 1, ...) to kzalloc this patch
reverses the logic around 'buf'.

Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>