vfs: set cn_namelen when handling degenerate lookups

Turns out execve looks at it to store binary name, but in order to
trigger the problem one has to be trying to exec '/'. As is the value
would be left uninitialized (or rather set to -1 on debug kernels).

Fixes: 56244d35741a62e7 ("vfs: hoist degenerate path lookups out of the
loop")