block: handle the null_mapped flag correctly in blk_rq_map_user_iov

The tape drivers (and the sg driver in a special case that doesn't matter
here) use the null_mapped flag to tell blk_rq_map_user to not copy around
any data into or out of the bounce buffers.  blk_rq_map_user_iov never
got that treatment, which didn't matter until I refactored blk_rq_map_user
to be implemented in terms of blk_rq_map_user_iov.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: ddad8dd0a162 ("block: use blk_rq_map_user_iov to implement blk_rq_map_user")
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-map.c

@@ -93,6 +93,9 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 	if (IS_ERR(bio))
 		return PTR_ERR(bio);
 
+	if (map_data && map_data->null_mapped)
+		bio->bi_flags |= (1 << BIO_NULL_MAPPED);
+
 	if (bio->bi_iter.bi_size != iter->count) {
 		/*
 		 * Grab an extra reference to this bio, as bio_unmap_user()