coda_venus_readdir(): use offsetof()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/coda/dir.c b/fs/coda/dir.c
index cd55323..9d52897 100644
--- a/fs/coda/dir.c
+++ b/fs/coda/dir.c
@@ -449,8 +449,7 @@
 	struct file *host_file;
 	struct dentry *de;
 	struct venus_dirent *vdir;
-	unsigned long vdir_size =
-	    (unsigned long)(&((struct venus_dirent *)0)->d_name);
+	unsigned long vdir_size = offsetof(struct venus_dirent, d_name);
 	unsigned int type;
 	struct qstr name;
 	ino_t ino;