o Use M_NOWAIT instead of M_WAITOK. It's assumed. This is a good area

for control by a flag.

o Add busdma_mem_get_seg_addr() and busdma_mem_get_seg_busaddr() so that

drivers can obtain address information about a segment. To be added are
busdma_mem_get_seg_paddr() and busdma_mem_get_seg_size().

XXX: I assume allocations for 1 segment only. The linked list approach

is probably not how I want to productize.