In contrast to FreeBSD 4 and 5, our slab allocator does hand out cross-page

In contrast to FreeBSD 4 and 5, our slab allocator does hand out cross-page
allocations. This broke bus_dmamem_alloc, which depends on allocation of
size < PAGE_SIZE to not cross pages.

As a temporary workaround, bus_dmamem_alloc checks explicitly wether an
allocation crossed page boundaries and retries the allocation with size
rounded up to the next power-of-two.