mtd: rawnand: nandsim: Replace overflow check with kzalloc to single kcalloc

Instead of self-checking overflow and allocating an array of specific size
by counting the total required space handy, we already have existed kernel
API which responses for all these works.

Signed-off-by: RinHizakura <s921975628@gmail.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211225100607.118932-1-s921975628@gmail.com