skd: reorder construct/destruct code

Reorder placement of skd_construct(), skd_cons_sg_list(), skd_destruct()
and skd_free_sg_list() functions. Then remove no longer needed function
prototypes.

Cc: Akhil Bhansali <abhansali@stec-inc.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>