Use busdma unconditionally in iflib

- Remove the complex mechanism to choose between using busdma
and raw pmap_kextract at runtime.   The reduced complexity makes
the code easier to read and maintain.

- Fix a bug in the small packet receive path where clusters were
repeatedly mapped but never unmapped. We now store the cluster's
bus address and avoid re-mapping the cluster each time a small
packet is received.

This patch fixes bugs I've seen where ixl(4) will not even
respond to ping without seeing DMAR faults.

I see a small improvement (14%) on packet forwarding tests using
a Haswell based Xeon E5-2697 v3.  Olivier sees a small
regression (-3% to -6%) with lower end hardware.