x86/intel_rdt: Add framework for better RDT UI diagnostics

Commands are given to the resctrl file system by making/removing
directories, or by writing to files.  When something goes wrong
the user is generally left wondering why they got:

   bash: echo: write error: Invalid argument

Add a new file "last_cmd_status" to the "info" directory that
will give the user some better clues on what went wrong.

Provide functions to clear and update last_cmd_status which
check that we hold the rdtgroup_mutex.

[ tglx: Made last_cmd_status static and folded back the hunk from patch 3
  	which replaces the open coded access to last_cmd_status with the
  	accessor function ]

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vikas Shivappa <vikas.shivappa@intel.com>
Cc: Boris Petkov <bp@suse.de>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lkml.kernel.org/r/edc4e0e9741eee89bba569f0021b1b2662fd9508.1506382469.git.tony.luck@intel.com

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