scsi: target: xen-scsiback: Convert to new submission API

target_submit_cmd_map_sgls() is being removed, so convert xen to the new
submission API. This has it use target_init_cmd(), target_submit_prep(), or
target_submit() because we need to have LIO core map sgls which is now done
in target_submit_prep(). target_init_cmd() will never fail for xen because
it does its own sync during session shutdown, so we can remove that code.

Note: xen never calls target_stop_session() so target_submit_cmd_map_sgls()
never failed (in the new API target_init_cmd() handles
target_stop_session() being called when cmds are being submitted). If it
were to have used target_stop_session() and got an error, we would have hit
a refcount bug like xen and usb, because it does:

    if (rc < 0) {
            transport_send_check_condition_and_sense(se_cmd,
                            TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE, 0);
            transport_generic_free_cmd(se_cmd, 0);
    }

transport_send_check_condition_and_sense() calls queue_status which calls
scsiback_cmd_done->target_put_sess_cmd. We do an extra
transport_generic_free_cmd() call above which would have dropped the
refcount to -1 and the refcount code would spit out errors.

Link: https://lore.kernel.org/r/20210227170006.5077-13-michael.christie@oracle.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>