This stops busdma bounce making assumptions about alignment of malloc(9)
results, which are no longer true.
Also add assert that the result of malloc_aligned() fits into single
page, which is the assumption of the code.
Reported by: dim
Reviewed by: andrew, jah, markj
Tested by: pho
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28147
f3ea417f96