arm64: busdma_bounce: fix BUS_DMA_ALLOCNOW for non-paged aligned sizes

For any size that isn't page-aligned, we end up not pre-allocating enough
for a single mapping because we truncate the size instead of rounding up to
make sure the last bit is accounted for, leaving us one page shy of what we
need to fulfill a request.

Differential Revision: https://reviews.freebsd.org/D22288