busdma: _bus_dmamap_addseg repaired

A recent change introduced a one-off error into a test allowing
coalescing chunks into segments. This fixes that error.

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.
Reported by: se
Reviewed by: se
Fixes: c606ab59e7f9 vm_extern: use standard address checkers everywhere
Differential Revision: https://reviews.freebsd.org/D33715

(cherry picked from commit f1e7a532d1ee23033fe0f3beb8eee80c412eeefe)