block: allow passing a NULL bdev to bio_alloc_clone/bio_init_clone

Device mapper wants to allocate a bio before knowing the device it
gets send to, so add explicit support for that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Link: https://lore.kernel.org/r/20220504142950.567582-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -761,13 +761,15 @@ static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
 	bio_set_flag(bio, BIO_CLONED);
 	if (bio_flagged(bio_src, BIO_THROTTLED))
 		bio_set_flag(bio, BIO_THROTTLED);
-	if (bio->bi_bdev == bio_src->bi_bdev &&
-	    bio_flagged(bio_src, BIO_REMAPPED))
-		bio_set_flag(bio, BIO_REMAPPED);
 	bio->bi_ioprio = bio_src->bi_ioprio;
 	bio->bi_iter = bio_src->bi_iter;
 
-	bio_clone_blkg_association(bio, bio_src);
+	if (bio->bi_bdev) {
+		if (bio->bi_bdev == bio_src->bi_bdev &&
+		    bio_flagged(bio_src, BIO_REMAPPED))
+			bio_set_flag(bio, BIO_REMAPPED);
+		bio_clone_blkg_association(bio, bio_src);
+	}
 
 	if (bio_crypt_clone(bio, bio_src, gfp) < 0)
 		return -ENOMEM;