KVM: x86/mmu: add missing update to max_mmu_rmap_size

The update to statistic max_mmu_rmap_size is unintentionally removed by
commit 4293ddb7 ("KVM: x86/mmu: Remove redundant spte present check
in mmu_set_spte"). Add missing update to it or max_mmu_rmap_size will
always be nonsensical 0.

Fixes: 4293ddb7

 ("KVM: x86/mmu: Remove redundant spte present check in mmu_set_spte")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Message-Id: <20220907080657.42898-1-linmiaohe@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>