KVM: x86: Drop pending_smi vs. INIT_RECEIVED check when setting MP_STATE

Allow userspace to set a vCPU's mp_state to INIT_RECEIVED in conjunction
with a pending SMI, as rejecting that combination could result in KVM
disallowing reflecting the output from KVM_GET_VCPU_EVENTS back into KVM
via KVM_SET_VCPU_EVENTS.

At the time the check was added, smi_pending could only be set in the
context of KVM_RUN, with the vCPU in the RUNNABLE state.  I.e. it was
impossible for KVM to save vCPU state such that userspace could see a
pending SMI for a vCPU in WFS.

That no longer holds true now that KVM processes requested SMIs during
KVM_GET_VCPU_EVENTS, e.g. if a vCPU receives an SMI while in WFS, and
then userspace saves vCPU state.

Note, this may partially re-open the user-triggerable WARN that was mostly
closed by commit 28bf2888 ("KVM: x86: fix user triggerable warning in
kvm_apic_accept_events()"), but that WARN can already be triggered in
several other ways, e.g. if userspace stuffs VMXON=1 after putting the
vCPU into WFS.  That issue will be addressed in an upcoming commit, in a
more robust fashion (hopefully).

Fixes: 1f7becf1 ("KVM: x86: get smi pending status correctly")
Link: https://lore.kernel.org/r/20250605195018.539901-2-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>