[PATCH] fix oops in namespace.c

A familar type of Oops: d_path() can return an error ENAMETOOLONG, and
if we fail to test a segfault occurs.

So we must test.  What we do is a different matter.  Rather arbitrarily
I return the string " (too long)" for use in /proc/mounts.

fs/namespace.c

@@ -217,6 +217,8 @@ static int show_vfsmnt(struct seq_file *m, void *v)
 	if (!path_buf)
 		return -ENOMEM;
 	path = d_path(mnt->mnt_root, mnt, path_buf, PAGE_SIZE);
+	if (IS_ERR(path))
+		path = " (too long)";
 
 	mangle(m, mnt->mnt_devname ? mnt->mnt_devname : "none");
 	seq_putc(m, ' ');