Don't bother trying to work around buffers which are not aligned on a cache
line boundary. It has never been 100% correct, and it can't work on SMP,
because nothing prevents another core from accessing data from an unrelated
buffer in the same cache line while we invalidated it. Just use bounce pages
instead.

Reviewed by:	ian
Approved by:	mux (mentor) (implicit)
Fix previous commit: both flags must be set.