s390/startup: round down "mem" option to page boundary

Make a usable value out of "mem" option once and for all. Kasan memory
allocator just takes memory_end or online memory size as allocation
base. If memory_end is not aligned paging structures allocated in kasan
end up unaligned as well. So this change fixes potential kasan crash
as well.

Fixes: 78333d1f ("s390/kasan: add support for mem= kernel parameter")
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>