[scsi] Make SCSI command issuing partially asynchronous

Move the icky call to step() from iscsi.c to scsi.c; this takes it at
least one step further away from where it really doesn't belong.

src/drivers/block/scsi.c

 #include <byteswap.h>
 #include <errno.h>
 #include <gpxe/blockdev.h>
+#include <gpxe/process.h>
 #include <gpxe/scsi.h>
 
 /** @file
 	/* Clear sense response code before issuing command */
 	command->sense_response = 0;
 
+	/* Flag command as in-progress */
+	command->rc = -EINPROGRESS;
+
 	/* Issue SCSI command */
 	if ( ( rc = scsi->command ( scsi, command ) ) != 0 ) {
-		/* Something went wrong with the issuing mechanism,
-		 * (rather than with the command itself)
-		 */
+		/* Something went wrong with the issuing mechanism */
+		DBG ( "SCSI %p " SCSI_CDB_FORMAT " err %s\n",
+		      scsi, SCSI_CDB_DATA ( command->cdb ), strerror ( rc ) );
+		return rc;
+	}
+
+	/* Wait for command to complete */
+	while ( command->rc == -EINPROGRESS )
+		step();
+	if ( ( rc = command->rc ) != 0 ) {
+		/* Something went wrong with the command execution */
 		DBG ( "SCSI %p " SCSI_CDB_FORMAT " err %s\n",
 		      scsi, SCSI_CDB_DATA ( command->cdb ), strerror ( rc ) );
 		return rc;

src/include/gpxe/iscsi.h

 	 * Set to NULL when command is complete.
 	 */
 	struct scsi_command *command;
-	/** SCSI command return code
-	 *
-	 * Set to -EINPROGRESS while command is processing.
-	 */
-	int rc;
 	/** Instant return code
 	 *
 	 * Set to a non-zero value if all requests should return

src/include/gpxe/scsi.h

 	uint8_t status;
 	/** SCSI sense response code */
 	uint8_t sense_response;
+	/** Command status code */
+	int rc;
 };
 
 /** A SCSI device */
 	 * @ret rc		Return status code
 	 *
 	 * Note that a successful return status code indicates only
-	 * that the SCSI command completed.  The caller must check the
-	 * status field in the command structure to see if, for
-	 * example, the device returned CHECK CONDITION or some other
-	 * non-success status code.
+	 * that the SCSI command was issued.  The caller must check
+	 * the status field in the command structure to see when the
+	 * command completes and whether, for example, the device
+	 * returned CHECK CONDITION or some other non-success status
+	 * code.
 	 */
 	int ( * command ) ( struct scsi_device *scsi,
 			    struct scsi_command *command );

src/net/tcp/iscsi.c

 static void iscsi_scsi_done ( struct iscsi_session *iscsi, int rc ) {
 
 	assert ( iscsi->tx_state == ISCSI_TX_IDLE );
+	assert ( iscsi->command != NULL );
 
+	iscsi->command->rc = rc;
 	iscsi->command = NULL;
-	iscsi->rc = rc;
 }
 
 /****************************************************************************
 		container_of ( scsi->backend, struct iscsi_session, refcnt );
 	int rc;
 
+	/* Abort immediately if we have a recorded permanent failure */
+	if ( iscsi->instant_rc )
+		return iscsi->instant_rc;
+
 	/* Record SCSI command */
 	iscsi->command = command;
 
-	/* Abort immediately if we have a recorded permanent failure */
-	if ( iscsi->instant_rc ) {
-		rc = iscsi->instant_rc;
-		goto done;
-	}
-
 	/* Issue command or open connection as appropriate */
 	if ( iscsi->status ) {
 		iscsi_start_command ( iscsi );
 	} else {
-		if ( ( rc = iscsi_open_connection ( iscsi ) ) != 0 )
-			goto done;
+		if ( ( rc = iscsi_open_connection ( iscsi ) ) != 0 ) {
+			iscsi->command = NULL;
+			return rc;
+		}
 	}
 
-	/* Wait for command to complete */
-	iscsi->rc = -EINPROGRESS;
-	while ( iscsi->rc == -EINPROGRESS )
-		step();
-	rc = iscsi->rc;
-
- done:
-	iscsi->command = NULL;
-	return rc;
+	return 0;
 }
 
 static int iscsi_detached_command ( struct scsi_device *scsi __unused,