busdma(9): Obey alignment constraint, disabled by default

busdma(9): Obey alignment constraint, disabled by default

- Obey alignment constraint
  Obtained-from: FreeBSD
- Correct "cross page boundary" check in bus_dmamem_alloc()
- Make sure that after kmalloc() retry, memory will not cross page
  boundary and is properly aligned.  If not, print backtrace; should
  panic though.
- Factor out BUS_DMA_KMALLOC(), so we don't duplicate the conditions
  in bus_dmamem_alloc() and bus_dmamem_free()
- Assert the input memory is properly aligned, if caller told us so