scsi: target: sbp_target: Convert to new submission API

target_submit_cmd() is now only for simple drivers that do their own sync
during shutdown and do not use target_stop_session(). It will never return
a failure, so we can remove that code from the driver.

Link: https://lore.kernel.org/r/20210227170006.5077-10-michael.christie@oracle.com
Cc: Chris Boot <bootc@bootc.net>
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>