socket while it is still in use.
priv->ctlsock is checked at the top of the function but without any
lock held, which means the control socket state may certainly change.
Add a similar protection to ngs_shutdown() even if a race is unlikely
to be experienced there.
Sponsored by: Sandvine Incorporated
Obtained from: Nima Misaghian @ Sandvine Incorporated
<nmisaghian at sandvine dot com>
MFC after: 10 days
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