Check the page table before TLB1 in pmap_kextract()

The vast majority of pmap_kextract() calls are looking for a physical memory
address, not a device address. By checking the page table first this saves
the formerly inevitable 64 (on e500mc and derivatives) iteration loop
through TLB1 in the most common cases.

Benchmarking this on the P5020 (e5500 core) yields a 300% throughput
improvement on dtsec(4) (115Mbit/s -> 460Mbit/s) measured with iperf.

Benchmarked on the P1022 (e500v2 core, 16 TLB1 entries) yields a 50%
throughput improvement on tsec(4) (~93Mbit/s -> 165Mbit/s) measured with
iperf.

MFC after: 1 week
Relnotes: Maybe (significant performance improvement)