Replace the remaining uses of vm_radix_node_page() by vm_radix_isleaf() and
vm_radix_topage(). This transformation eliminates some unnecessary
conditional branches from the inner loops of vm_radix_insert(),
vm_radix_lookup{,_ge,_le}(), and vm_radix_remove().

Simplify the control flow of vm_radix_lookup_{ge,le}().

Reviewed by: attilio (an earlier version)
Tested by: pho
Sponsored by: EMC / Isilon Storage Division