mips: fix busdma bugs introduced in merge

[dougm]

Summary

The change "vm_extern: use standard address checkers everywhere" introduced bugs, as well as features, into head. After it was introduced, but before those bugs were addressed, mips support was dropped from head, so those bugs were never fixed in head. Now, that change and those fixes have been merged into stable/13. This change fixes two of those bugs that were fixed everywhere else but never in mips.