x86: memtest: use pointers of equal type for comparison

Commit c9690998

 (x86: memtest: remove
64-bit division) introduced following compile warning:

arch/x86/mm/memtest.c: In function 'memtest':
arch/x86/mm/memtest.c:56: warning: comparison of distinct pointer types lacks a cast
arch/x86/mm/memtest.c:58: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>