read_file: Use read_full

At least NFS does not fulfill read requests > 1024 bytes at once.
Use read_full in read_file as read doesn't guarantee that the
whole request can be read at once.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

fs/fs.c

@@ -60,7 +60,8 @@ again:
 	if (fd < 0)
 		goto err_out;
 
-	if (read(fd, buf, s.st_size) < s.st_size)
+	ret = read_full(fd, buf, s.st_size);
+	if (ret < 0)
 		goto err_out1;
 
 	close(fd);