Commit 0ba99ca4838b ("block: Add warning for bi_next not NULL in
bio_endio()") breaks the dm driver. end_clone_bio() detects whether
or not a bio is the last bio associated with a request by checking
the .bi_next field. Commit 0ba99ca4838b clears that field before
end_clone_bio() has had a chance to inspect that field. Hence revert
commit 0ba99ca4838b.