mufs: keep track of read/written bytes in mufs_read()/mufs_write()

source/fs/mufs/tree.c

@@ -196,6 +196,7 @@ size_t mufs_read(struct mufs *fs, struct mufs_tree *tree, void *data,
 
 		buf += ret;
 		va += ret;
+		nbytes += ret;
 		len -= ret;
 	}
 
@@ -235,6 +236,7 @@ size_t mufs_write(struct mufs *fs, struct mufs_tree *tree, void *data,
 
 		buf += ret;
 		va += ret;
+		nbytes += ret;
 		len -= ret;
 	}