net, netns_xt: shrink netns_xt members

In case if kernel was compiled without ebtables support
there is no need to keep ebt_table pointers in netns_xt
structure.

Make it config dependent.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>