KVM: x86/mmu: Update page stats in __rmap_add()

Update the page stats in __rmap_add() rather than at the call site. This
will avoid having to manually update page stats when splitting huge
pages in a subsequent commit.

No functional change intended.

Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-17-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>