tcmu: clean up the scatter helper

Add some comments to make the scatter code to be more readable,
and drop unused arg to new_iov.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>