iscsi_iser: convert to use the data buffer accessors

- remove the unnecessary map_single path.

- convert to use the new accessors for the sg lists and the
parameters.

TODO: use scsi_for_each_sg().
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Erez Zilber <erezz@voltaire.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>