net: websockets: Fix ZFD_IOCTL_SET_LOCK handling

The websocket "socket" layer did not handle ZFD_IOCTL_SET_LOCK command,
and just forwarded it to the underlying socket. This overwritten the
mutex pointer used by the underlying socket, resulting in erroneous
behaviour in certain cases.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>