kill f_dentry uses

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/sync.c b/fs/sync.c
index bdc729d..01d9f18 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -154,7 +154,7 @@
 
 	if (!f.file)
 		return -EBADF;
-	sb = f.file->f_dentry->d_sb;
+	sb = f.file->f_path.dentry->d_sb;
 
 	down_read(&sb->s_umount);
 	ret = sync_filesystem(sb);