scatterlist: make sure sg_miter_next() doesn't return 0 sized mappings

Impact: fix not-so-critical but annoying bug

sg_miter_next() returns 0 sized mapping if there is an zero sized sg
entry in the list or at the end of each iteration.  As the users
always check the ->length field, this bug shouldn't be critical other
than causing unnecessary iteration.

Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>