KVM: x86: Unconditionally grab data from EDX:EAX in WRMSR fastpath

Always grab EDX:EAX in the WRMSR fastpath to deduplicate and simplify the
case statements, and to prepare for handling immediate variants of WRMSRNS
in the fastpath (the data register is explicitly provided in that case).
There's no harm in reading the registers, as their values are always
available, i.e. don't require VMREADs (or similarly slow operations).

No real functional change intended.

Cc: Xin Li <xin@zytor.com>
Link: https://lore.kernel.org/r/20250805190526.1453366-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -2157,18 +2157,16 @@ static int handle_fastpath_set_tscdeadline(struct kvm_vcpu *vcpu, u64 data)
 
 fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
 {
+	u64 data = kvm_read_edx_eax(vcpu);
 	u32 msr = kvm_rcx_read(vcpu);
-	u64 data;
 	bool handled;
 	int r;
 
 	switch (msr) {
 	case APIC_BASE_MSR + (APIC_ICR >> 4):
-		data = kvm_read_edx_eax(vcpu);
 		handled = !handle_fastpath_set_x2apic_icr_irqoff(vcpu, data);
 		break;
 	case MSR_IA32_TSC_DEADLINE:
-		data = kvm_read_edx_eax(vcpu);
 		handled = !handle_fastpath_set_tscdeadline(vcpu, data);
 		break;
 	default: