sysctl: net: call unregister_net_sysctl_table where needed

ctl_table_headers registered with register_net_sysctl_table should
have been unregistered with the equivalent unregister_net_sysctl_table

Signed-off-by: Lucian Adrian Grijincu <lucian.grijincu@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>