KVM: VMX: do not use uninitialized gfn_to_hva_cache

An uninitialized gfn_to_hva_cache has ghc->len == 0, which causes
the accessors to croak very loudly.  While a BUG_ON is definitely
_too_ loud and a bug on its own, there is indeed an issue of using
the caches in such a way that they could not have been initialized,
because ghc->gpa == 0 might match and thus kvm_gfn_to_hva_cache_init
would not be called.

For the vmcs12_cache, the solution is simply to invoke
kvm_gfn_to_hva_cache_init unconditionally: we already know
that the cache does not match the current VMCS pointer.
For the shadow_vmcs12_cache, there is no similar condition
that checks the VMCS link pointer, so invalidate the cache
on VMXON.

Fixes: cee66664 ("KVM: nVMX: Use a gfn_to_hva_cache for vmptrld")
Acked-by: David Woodhouse <dwmw@amazon.co.uk>
Reported-by:  <syzbot+7b7db8bb4db6fd5e157b@syzkaller.appspotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>