MFC: r212663

MFC: r212663

- Update the comment in swi_vm() regarding busdma bounce buffers; it's
  unlikely that support for these ever will be implemented on sparc64 as
  the IOMMUs are able to translate to up to the maximum physical address
  supported by the respective machine, bypassing the IOMMU is affected
  by hardware errata and being able to support DMA engines which cannot
  do at least 32-bit DMA does not justify the costs.
- The page zeroing in uma_small_alloc() may use the VIS-based block zero
  function so take advantage of it.

git-svn-id: svn://svn.freebsd.org/base/stable/8@213014 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f