Eliminate 1 XX_VirtToPhys() and 2 XX_PhysToVirt() calls from if_dtsec(4)

XX_VirtToPhys(), by way of pmap_kextract(), is an expensive operation.
Profiling via dtrace during a series of iperf tests I found 16111 / 30432
stack frames were located in mmu_booke_kextract(), so eliminating this
expensive call should improve performance slightly. XX_PhysToVirt() is not
as expensive, but redundant calls in this context is wasteful.