md: multipath: don't hardcopy bio in .make_request path

Inside multipath_make_request(), multipath maps the incoming
bio into low level device's bio, but it is totally wrong to
copy the bio into mapped bio via '*mapped_bio = *bio'. For
example, .__bi_remaining is kept in the copy, especially if
the incoming bio is chained to via bio splitting, so .bi_end_io
can't be called for the mapped bio at all in the completing path
in this kind of situation.

This patch fixes the issue by using clone style.

Cc: stable@vger.kernel.org (v3.14+)
Reported-and-tested-by: Andrea Righi <righi.andrea@gmail.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/multipath.c

@@ -129,7 +129,9 @@ static void multipath_make_request(struct mddev *mddev, struct bio * bio)
 	}
 	multipath = conf->multipaths + mp_bh->path;
 
-	mp_bh->bio = *bio;
+	bio_init(&mp_bh->bio);
+	__bio_clone_fast(&mp_bh->bio, bio);
+
 	mp_bh->bio.bi_iter.bi_sector += multipath->rdev->data_offset;
 	mp_bh->bio.bi_bdev = multipath->rdev->bdev;
 	mp_bh->bio.bi_rw |= REQ_FAILFAST_TRANSPORT;