KVM: x86/mmu: Deduplicate rmap freeing

Small code deduplication. No functional change expected.

Reviewed-by: David Hildenbrand <david@redhat.com>

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-2-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>