MIPS: KVM: Fix memory leak on VCPU

kvm_arch_vcpu_free() is called in 2 code paths:

1) kvm_vm_ioctl()
       kvm_vm_ioctl_create_vcpu()
           kvm_arch_vcpu_destroy()
               kvm_arch_vcpu_free()
2) kvm_put_kvm()
       kvm_destroy_vm()
           kvm_arch_destroy_vm()
               kvm_mips_free_vcpus()
                   kvm_arch_vcpu_free()

Neither of the paths handles VCPU free. We need to do it in
kvm_arch_vcpu_free() corresponding to the memory allocation in
kvm_arch_vcpu_create().

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Cc: stable@vger.kernel.org
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>