x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

In a context switch from a task that is detecting split locks to one that
is not (or vice versa) we need to update the TEST_CTRL MSR. Currently this
is done with the common sequence:

        read the MSR
	flip the bit
	write the MSR
in order to avoid changing the value of any reserved bits in the MSR.

Cache unused and reserved bits of TEST_CTRL MSR with SPLIT_LOCK_DETECT bit
cleared during initialization, so we can avoid an expensive RDMSR
instruction during context switch.

Suggested-by: Sean Christopherson <[email protected]>
Originally-by: Tony Luck <[email protected]>
Signed-off-by: Xiaoyao Li <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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