arch/x86/mm/init_64.c: early_memtest(): fix types

fix this warning:

arch/x86/mm/init_64.c: In function 'early_memtest':
arch/x86/mm/init_64.c:524: warning: passing argument 2 of 'find_e820_area_size' from incompatible pointer type

Signed-off-by: Ingo Molnar <mingo@elte.hu>