x86/intel_rdt: Implement show_options() for resctrlfs

Implement show_options() callback for intel resource control filesystem
to expose the active mount options in /proc/

Signed-off-by: Shaohua Li <shli@fb.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/7dce7c1886ac9289442d254ea18322c92bd968da.1480717072.git.shli@fb.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>