scsi: target: srpt: Convert to new submission API

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

srpt uses target_stop_session() to sync session shutdown with LIO core, so
we use target_init_cmd()/target_submit_prep()/target_submit(), because
target_init_cmd() will detect the target_stop_session() call and return an
error.

Link: https://lore.kernel.org/r/20210227170006.5077-6-michael.christie@oracle.com


Cc: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>