refactor loading of dmamaps.

bus_dmama_load and bus_dmamap_load mbuf figure out the physical
addresses of the memory theyre given and then hand it to
_bus_dmamap_load_paddr to store in the dmamaps sg lists.

unfortunately bus_dmamap_load_mbuf assumes it is only given memory
from the kernels direct mapped region, and blindly translates
anything its given into phys addresses to hand to _load_paddr.

i recently committed change to pool asking them to allocate large
pages, which meant uvm allocated mbufs outside the direct map, which
meant bus_dmamap_load_mbuf was handing out bogus physical addresses.
the pool change got backed out until i could debug this.

now _load and _load_mbuf now call _bus_dmamap_load_vaddr for every
buffer theyve been given, which properly determines if the addresses
are in the direct map or via the tlb. _load_vaddr then feeds the
physical addresses into _bus_dmamap_load_paddr to store them in the
dmamap.

tldr; _load_mbuf doesnt make naive assumptions about its addresses
now.

ok miod@ kettenis@

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