jfs: 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>

fs/jfs/namei.c

@@ -1600,7 +1600,7 @@ static int jfs_ci_compare(const struct dentry *parent,
 
 static int jfs_ci_revalidate(struct dentry *dentry, struct nameidata *nd)
 {
-	if (nd->flags & LOOKUP_RCU)
+	if (nd && nd->flags & LOOKUP_RCU)
 		return -ECHILD;
 	/*
 	 * This is not negative dentry. Always valid.