KVM: VMX: fix vmwrite to invalid VMCS

fpu_activate is called outside of vcpu_load(), which means it should not
touch VMCS, but fpu_activate needs to.  Avoid the call by moving it to a
point where we know that the guest needs eager FPU and VMCS is loaded.

This will get rid of the following trace

 vmwrite error: reg 6800 value 0 (err 1)
  [<ffffffff8162035b>] dump_stack+0x19/0x1b
  [<ffffffffa046c701>] vmwrite_error+0x2c/0x2e [kvm_intel]
  [<ffffffffa045f26f>] vmcs_writel+0x1f/0x30 [kvm_intel]
  [<ffffffffa04617e5>] vmx_fpu_activate.part.61+0x45/0xb0 [kvm_intel]
  [<ffffffffa0461865>] vmx_fpu_activate+0x15/0x20 [kvm_intel]
  [<ffffffffa0560b91>] kvm_arch_vcpu_create+0x51/0x70 [kvm]
  [<ffffffffa0548011>] kvm_vm_ioctl+0x1c1/0x760 [kvm]
  [<ffffffff8118b55a>] ? handle_mm_fault+0x49a/0xec0
  [<ffffffff811e47d5>] do_vfs_ioctl+0x2e5/0x4c0
  [<ffffffff8127abbe>] ? file_has_perm+0xae/0xc0
  [<ffffffff811e4a51>] SyS_ioctl+0xa1/0xc0
  [<ffffffff81630949>] system_call_fastpath+0x16/0x1b

(Note: we also unconditionally activate FPU in vmx_vcpu_reset(), so the
 removed code added nothing.)

Fixes: c447e76b ("kvm/fpu: Enable eager restore kvm FPU for MPX")
Cc: <stable@vger.kernel.org>
Reported-by: Vlastimil Holer <vlastimil.holer@gmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>