NVME: Multiple busdma related fixes.

NVME: Multiple busdma related fixes.
- in nvme_qpair_process_completions() do dma sync before completion buffer
  is used.
- in nvme_qpair_submit_tracker(), don't do explicit wmb() also for arm
  and arm64. Bus_dmamap_sync() on these architectures is sufficient to ensure
  that all CPU stores are visible to external (including DMA) observers.
- Allocate completion buffer as BUS_DMA_COHERENT. On not-DMA coherent systems,
  buffers continuously owned (and accessed) by DMA must be allocated with this
  flag. Note that BUS_DMA_COHERENT flag is no-op on DMA coherent systems
  (or coherent buses in mixed systems).

MFC after:	4 weeks
Reviewed by:	mav, imp
Differential Revision: https://reviews.freebsd.org/D27446