Add more safeguards to protect against misinterpreting a chain entry
as a normal scatterlist and vice-versa.
* Make sure the entry isn't a chain when assigning and reading a
normal sg.
* Clear offset and length when chaining.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>