x86/umwait: Add sysfs interface to control umwait C0.2 state

x86/umwait: Add sysfs interface to control umwait C0.2 state

C0.2 state in umwait and tpause instructions can be enabled or disabled
on a processor through IA32_UMWAIT_CONTROL MSR register.

By default, C0.2 is enabled and the user wait instructions results in
lower power consumption with slower wakeup time.

But in real time systems which require faster wakeup time although power
savings could be smaller, the administrator needs to disable C0.2 and all
umwait invocations from user applications use C0.1.

Create a sysfs interface which allows the administrator to control C0.2
state during run time.

Andy Lutomirski suggested to turn off local irqs before writing the MSR to
ensure the cached control value is not changed by a concurrent sysfs write
from a different CPU via IPI.

[ tglx: Simplified the update logic in the write function and got rid of
  	all the convoluted type casts. Added a shared update function and
	made the namespace consistent. Moved the sysfs create invocation.
	Massaged changelog ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: "Borislav Petkov" <bp@alien8.de>
Cc: "H Peter Anvin" <hpa@zytor.com>
Cc: "Andy Lutomirski" <luto@kernel.org>
Cc: "Peter Zijlstra" <peterz@infradead.org>
Cc: "Ravi V Shankar" <ravi.v.shankar@intel.com>
Link: https://lkml.kernel.org/r/1560994438-235698-4-git-send-email-fenghua.yu@intel.com