x86/hyperv: Fix unused variable 'msr_val' warning in hv_qlock_wait

x86/hyperv: Fix unused variable 'msr_val' warning in hv_qlock_wait

Fixes the following W=1 kernel build warning(s):
arch/x86/hyperv/hv_spinlock.c:28:16: warning: variable ‘msr_val’ set but not used [-Wunused-but-set-variable]
  unsigned long msr_val;

As Hypervisor Top-Level Functional Specification states in chapter 7.5
Virtual Processor Idle Sleep State, "A partition which possesses the
AccessGuestIdleMsr privilege (refer to section 4.2.2) may trigger entry
into the virtual processor idle sleep state through a read to the
hypervisor-defined MSR HV_X64_MSR_GUEST_IDLE".

That means only a read of the MSR is necessary. The returned value
msr_val is not used. Cast it to void to silence this warning.

Reference:
https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xu Yihang <xuyihang@huawei.com>
Link: https://lore.kernel.org/r/20210323024302.174434-1-xuyihang@huawei.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>