nvme: add comment for unaligned "fake" nqn

Current "fake" nqn field is "nqn.2014.08.org.nvmexpress:", it is
not aligned with the canonical version for history reasons.

Signed-off-by: Linjun Bao <meljbao@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>