BugLink: http://bugs.launchpad.net/bugs/1531539

This is the counter part to nvme_map_data.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit d4f6c3ab)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>