ns proc: Return -ENOENT for a nonexistent /proc/self/ns/ entry.

Spotted-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index f18d6d5..781dec5 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -161,6 +161,7 @@
 		if (!memcmp(dentry->d_name.name, (*entry)->name, len))
 			break;
 	}
+	error = ERR_PTR(-ENOENT);
 	if (entry > last)
 		goto out;