KVM: x86: pass struct kvm_mmu_page to gfn_to_rmap

This is always available (with one exception in the auditing code),
and with the same auditing exception the level was coming from
sp->role.level.

Later, the spte's role will also be used to look up the right memslots
array.

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>