Revert "kvm: x86: Use task structs fpu field for user"

This reverts commit 240c35a378
("kvm: x86: Use task structs fpu field for user", 2018-11-06).
The commit is broken and causes QEMU's FPU state to be destroyed
when KVM_RUN is preempted.

Fixes: 240c35a378 ("kvm: x86: Use task structs fpu field for user")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -607,15 +607,16 @@ struct kvm_vcpu_arch {
 
 	/*
 	 * QEMU userspace and the guest each have their own FPU state.
-	 * In vcpu_run, we switch between the user, maintained in the
-	 * task_struct struct, and guest FPU contexts. While running a VCPU,
-	 * the VCPU thread will have the guest FPU context.
+	 * In vcpu_run, we switch between the user and guest FPU contexts.
+	 * While running a VCPU, the VCPU thread will have the guest FPU
+	 * context.
 	 *
 	 * Note that while the PKRU state lives inside the fpu registers,
 	 * it is switched out separately at VMENTER and VMEXIT time. The
 	 * "guest_fpu" state here contains the guest FPU context, with the
 	 * host PRKU bits.
 	 */
+	struct fpu user_fpu;
 	struct fpu *guest_fpu;
 
 	u64 xcr0;