target: pass sense_reason as a return value

Pass the sense reason as an explicit return value from the I/O submission
path instead of storing it in struct se_cmd and using negative return
values.  This cleans up a lot of the code pathes, and with the sparse
annotations for the new sense_reason_t type allows for much better
error checking.

(nab: Convert spc_emulate_modesense + spc_emulate_modeselect to use
      sense_reason_t with Roland's MODE SELECT changes)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/vhost/tcm_vhost.c

@@ -541,10 +541,6 @@ static void tcm_vhost_submission_work(struct work_struct *work)
 
 	if (tv_cmd->tvc_sgl_count) {
 		sg_ptr = tv_cmd->tvc_sgl;
-		/*
-		 * For BIDI commands, pass in the extra READ buffer
-		 * to transport_generic_map_mem_to_cmd() below..
-		 */
 /* FIXME: Fix BIDI operation in tcm_vhost_submission_work() */
 #if 0
 		if (se_cmd->se_cmd_flags & SCF_BIDI) {