x86/intel_rdt: Switch to bitmap_zalloc()

Switch to bitmap_zalloc() to show clearly what is allocated. Besides that
it returns a pointer of bitmap type instead of opaque void *.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Fenghua Yu <fenghua.yu@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20180830115039.63430-1-andriy.shevchenko@linux.intel.com