clocksource: hyper-v: Use TSC PFN getter to map vvar page

Instead of converting the virtual address to physical directly.

This is a precursor patch for the upcoming support for TSC page mapping into
Microsoft Hypervisor root partition, where TSC PFN will be defined by the
hypervisor and thus can't be obtained by linear translation of the physical
address.

Signed-off-by: Stanislav Kinsburskiy <stanislav.kinsburskiy@gmail.com>
CC: Andy Lutomirski <luto@kernel.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: Borislav Petkov <bp@alien8.de>
CC: Dave Hansen <dave.hansen@linux.intel.com>
CC: x86@kernel.org
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: "K. Y. Srinivasan" <kys@microsoft.com>
CC: Haiyang Zhang <haiyangz@microsoft.com>
CC: Wei Liu <wei.liu@kernel.org>
CC: Dexuan Cui <decui@microsoft.com>
CC: Daniel Lezcano <daniel.lezcano@linaro.org>
CC: linux-kernel@vger.kernel.org
CC: linux-hyperv@vger.kernel.org
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
Link: https://lore.kernel.org/r/166749833939.218190.14095015146003109462.stgit@skinsburskii-cloud-desktop.internal.cloudapp.net


Signed-off-by: Wei Liu <wei.liu@kernel.org>