KVM: x86: Propagate AMD's IbrsSameMode to the guest

If IBRS provides same mode (kernel/user or host/guest) protection on the
host, then by definition it also provides same mode protection in the
guest. In fact, all different modes from the guest's perspective are the
same mode from the host's perspective anyway.

Propagate IbrsSameMode to the guests.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20250221163352.3818347-3-yosry.ahmed@linux.dev
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/cpuid.c

@@ -1093,6 +1093,7 @@ void kvm_set_cpu_caps(void)
 		F(AMD_SSB_NO),
 		F(AMD_STIBP),
 		F(AMD_STIBP_ALWAYS_ON),
+		F(AMD_IBRS_SAME_MODE),
 		F(AMD_PSFD),
 		F(AMD_IBPB_RET),
 	);