Check the return values of contigmalloc(9) as well as bus_dma(9)

functions and stop attaching of dcons(4) and dcons_crom(4) if
they indicate failure. This fixes a panic seen on sparc64 machines
with no free physical memory in the requested 32-bit region but
still doesn't make dcons(4)/dcons_crom(4) these work. I think
the latter can be fixed by simply specifying ~0UL as the upper
limit for contigmalloc(9) and letting the bounce pages and the
IOMMU respectively handle limitations of the DMA engine. I didn't
want to change that without the consensus of simokawa@ though,
who unfortunately didn't reply so far.

MFC after:	1 week

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