KVM: x86/mmu: only perform eager page splitting on valid roots

Eager page splitting is an optimization; it does not have to be performed on
invalid roots.  It is also the only case in which a reader might acquire
a reference to an invalid root, so after this change we know that readers
will skip both dying and invalid roots.

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