pstore/ram: Fix crash when setting number of cpus to an odd number

In the Linux kernel, the following vulnerability has been resolved:

pstore/ram: Fix crash when setting number of cpus to an odd number

When the number of cpu cores is adjusted to 7 or other odd numbers,

the zone size will become an odd number.

The address of the zone will become:

addr of zone0 = BASE

addr of zone1 = BASE + zone_size

addr of zone2 = BASE + zone_size*2

...

The address of zone1/3/5/7 will be mapped to non-alignment va.

Eventually crashes will occur when accessing these va.

So, use ALIGN_DOWN() to make sure the zone size is even

to avoid this bug.