x86/split_lock: Rework the initialization flow of split lock detection

Current initialization flow of split lock detection has following issues:

1. It assumes the initial value of MSR_TEST_CTRL.SPLIT_LOCK_DETECT to be
   zero. However, it's possible that BIOS/firmware has set it.

2. X86_FEATURE_SPLIT_LOCK_DETECT flag is unconditionally set even if
   there is a virtualization flaw that FMS indicates the existence while
   it's actually not supported.

Rework the initialization flow to solve above issues. In detail, explicitly
clear and set split_lock_detect bit to verify MSR_TEST_CTRL can be
accessed, and rdmsr after wrmsr to ensure bit is cleared/set successfully.

X86_FEATURE_SPLIT_LOCK_DETECT flag is set only when the feature does exist
and the feature is not disabled with kernel param "split_lock_detect=off"

On each processor, explicitly updating the SPLIT_LOCK_DETECT bit based on
sld_sate in split_lock_init() since BIOS/firmware may touch it.

Originally-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200325030924.132881-2-xiaoyao.li@intel.com

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