in particular, do not handle deferred DMA map load operations at all.
Any error, and especially EINPROGRESS, is treated as a hard error and
typically abort the current operation. The fact that the busdma code
queues the load operation for when resources (i.e. bounce buffers in
this particular case) are available makes this especially problematic.
Bounce buffering, unlike what the PR synopsis would suggest, works
fine.
While on the subject, properly implement swi_vm().
PR: 147502
MFC after: 1 week
f635c047c5