nvme: Pass pointers, not dma addresses, to nvme_get/set_features()

Any user I can imagine that needs a buffer at all will want to pass
a pointer directly.  There are no currently callers that use
buffers, so this change is painless, and it will make it much easier
to start using features that use buffers (e.g. APST).

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Tested-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>