scsi: target: tcm_loop: Convert to new submission API

target_submit_cmd_map_sgls() is being removed, so convert loop to
the new submission API.

Even though loop does its own shutdown sync, this has loop use
target_init_cmd()/target_submit_prep()/target_submit() since it needed to
map sgls and in the next patches it will use the API to use LIO's
workqueue.

Link: https://lore.kernel.org/r/20210227170006.5077-9-michael.christie@oracle.com
Tested-by: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>