libstorage: Fix ioctl() handling on fs

- return devctl error only via output argument,
- return -ENOTTY when no devctl handler is provided to return valid errno from ioctl()

JIRA: RTOS-525

libstorage/fs.c

@@ -75,10 +75,12 @@ void storage_fsHandler(void *data, msg_t *msg)
 
 		case mtDevCtl:
 			if (fs->ops->devctl == NULL) {
-				msg->o.io.err = -ENOSYS;
+				/* FIXME this error passing works by accident on ioctl(),
+				 * there's no dedicated error field for devctl. */
+				msg->o.io.err = -ENOTTY; /* To return valid errno on ioctl() */
 				break;
 			}
-			(void)fs->ops->devctl(fs->info, &msg->i.io.oid, msg->i.raw, msg->o.raw);
+			fs->ops->devctl(fs->info, &msg->i.io.oid, msg->i.raw, msg->o.raw);
 			break;
 
 		case mtCreate:

libstorage/include/storage/fs.h

@@ -32,7 +32,7 @@ typedef struct {
 	int (*setattr)(void *info, oid_t *oid, int type, long long attr, void *data, size_t len);
 	int (*getattr)(void *info, oid_t *oid, int type, long long *attr);
 	int (*truncate)(void *info, oid_t *oid, size_t size);
-	int (*devctl)(void *info, oid_t *oid, const void *in, void *out);
+	void (*devctl)(void *info, oid_t *oid, const void *in, void *out);
 
 	int (*create)(void *info, oid_t *oid, const char *name, oid_t *dev, unsigned mode, int type, oid_t *res);
 	int (*destroy)(void *info, oid_t *oid);