x86/hyperv: Move TSC reading method to asm/mshyperv.h

As a preparation to making Hyper-V TSC page suitable for vDSO move
the TSC page reading logic to asm/mshyperv.h. While on it, do the
following:

- Document the reading algorithm.
- Simplify the code a bit.
- Add explicit READ_ONCE() to not rely on 'volatile'.
- Add explicit barriers to prevent re-ordering (we need to read sequence
  strictly before and after)
- Use mul_u64_u64_shr() instead of assembly, gcc generates a single 'mul'
  instruction on x86_64 anyway.

[ tglx: Simplified the loop ]

Signed-off-by: Vitaly Kuznetsov <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: Dexuan Cui <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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