budsma: fix boundary alignment test

[dougm]

Summary

c606ab59e7f9 vm_extern: use standard address checkers everywhere

broke a check in _bus_dmamap_addseg on many architectures. This change makes it clear that it is not a particular range that is being boundary-checked, but the proposed union of the two adjacent ranges.

[se]

Looks good to me:

increase current segment by sgsize bytes if

• curraddr is contiguous to the end of the segment,
• adding sgsize will not cause maxsegsz to be exceeded, and
• start address plus increased segment length will still pass the boundary check.