scsi: sg: fix static checker warning in sg_is_valid_dxfer

dxfer_len is an unsigned int and we always assign a value > 0 to it, so
it doesn't make any sense to check if it is < 0. We can't really check
dxferp as well as we have both NULL and not NULL cases in the possible
call paths.

So just return true for SG_DXFER_FROM_DEV transfer in
sg_is_valid_dxfer().

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reported-by: Colin Ian King <colin.king@canonical.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -759,8 +759,11 @@ static bool sg_is_valid_dxfer(sg_io_hdr_t *hp)
 			return false;
 		return true;
 	case SG_DXFER_FROM_DEV:
-		if (hp->dxfer_len < 0)
-			return false;
+		/*
+		 * for SG_DXFER_FROM_DEV we always set dxfer_len to > 0. dxferp
+		 * can either be NULL or != NULL so there's no point in checking
+		 * it either. So just return true.
+		 */
 		return true;
 	case SG_DXFER_TO_DEV:
 	case SG_DXFER_TO_FROM_DEV: