dm: use bio_uninit instead of bio_disassociate_blkg

bio_uninit is the proper API to clean up a BIO that has been allocated
on stack or inside a structure that doesn't come from the BIO allocator.
Switch dm to use that instead of bio_disassociate_blkg, which really is
an implementation detail.  Note that the bio_uninit calls are also moved
to the two callers of __send_empty_flush, so that they better pair with
the bio_init calls used to initialize them.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1443,9 +1443,6 @@ static int __send_empty_flush(struct clone_info *ci)
 	BUG_ON(bio_has_data(ci->bio));
 	while ((ti = dm_table_get_target(ci->map, target_nr++)))
 		__send_duplicate_bios(ci, ti, ti->num_flush_bios, NULL);
-
-	bio_disassociate_blkg(ci->bio);
-
 	return 0;
 }
 
@@ -1633,6 +1630,7 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md,
 		ci.bio = &flush_bio;
 		ci.sector_count = 0;
 		error = __send_empty_flush(&ci);
+		bio_uninit(ci.bio);
 		/* dec_pending submits any data associated with flush */
 	} else if (op_is_zone_mgmt(bio_op(bio))) {
 		ci.bio = bio;
@@ -1707,6 +1705,7 @@ static blk_qc_t __process_bio(struct mapped_device *md, struct dm_table *map,
 		ci.bio = &flush_bio;
 		ci.sector_count = 0;
 		error = __send_empty_flush(&ci);
+		bio_uninit(ci.bio);
 		/* dec_pending submits any data associated with flush */
 	} else {
 		struct dm_target_io *tio;