KVM: x86: fix size of x86_fpu_cache objects

The memory allocation in b666a4b6

 ("kvm: x86: Dynamically allocate
guest_fpu", 2018-11-06) is wrong, there are other members in struct fpu
before the fpregs_state union and the patch should be doing something
similar to the code in fpu__init_task_struct_size.  It's enough to run
a guest and then rmmod kvm to see slub errors which are actually caused
by memory corruption.

For now let's revert it to sizeof(struct fpu), which is conservative.
I have plans to move fsave/fxsave/xsave directly in KVM, without using
the kernel FPU helpers, and once it's done, the size of the object in
the cache will be something like kvm_xstate_size.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>