nvme: set dma alignment to dword

The nvme specification only requires qword alignment for segment
descriptors, and the driver already guarantees that. The spec has always
allowed user data to be dword aligned, which is what the queue's
attribute is for, so relax the alignment requirement to that value.

While we could allow byte alignment for some controllers when using
SGLs, we still need to support PRP, and that only allows dword.

Fixes: 3b2a1ebc

 ("nvme: set dma alignment to qword")
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>