netfilter: x_tables: fix net namespace leak when reading /proc/net/xxx_tables_names

The seq_open_net() call should be accompanied with seq_release_net() one.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>