multisegment bus_dmamem_alloc()

[Mark Tinguely]

Are there any other devices other than bktr(4) that could benefit
from a multisegment bus_dmamem_alloc(9) routine? The old USB system
comes to mind, but I have not studied the new USB. There may be others.

NetBSD only partially implements this (if the number page allocated > number
of segments, they punt and do a totally contiguous allocation - IMO, they
could fix this pretty easy though).

I know chages to contigmalloc() is a tricky subject. I was hoping
there are enough drivers that would provide enough merits to change
such an important part of the system. If not, please forgive this noise.

--Mark Tinguely