JFFS2: Change scansize to match linux kernel

The scan code is similar to the linux kernel, but the kernel defines a much
smaller size to scan through before deciding a sector is blank. Assuming
that what is in the kernel is OK, make these two match.

On its own, this change makes no difference to scanning of any sectors
which have a clean marker at the beginning, since the entire sector is not
blank.

Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>