fuse: fix d_revalidate oopsen on NFS exports

can't blindly check nd->flags in ->d_revalidate()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 83543b5..8bd0ef9 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -158,7 +158,7 @@
 {
 	struct inode *inode;
 
-	if (nd->flags & LOOKUP_RCU)
+	if (nd && nd->flags & LOOKUP_RCU)
 		return -ECHILD;
 
 	inode = entry->d_inode;