mtd-tests: nandbiterrs: Store the chunks size in an intermediate variable

mtd-tests: nandbiterrs: Store the chunks size in an intermediate variable

'pagesize' is used for two purposes:
- accessing the size of a page
- getting the size of the test buffer, which happen to be the size of a
  page for now.
Use an intermediate variable when getting the size of the test buffer,
as we will later increase its size.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>