net/netfilter/x_tables.c: use __seq_open_private()

Reduce boilerplate code by using __seq_open_private() instead of seq_open()
in xt_match_open() and xt_target_open().

Signed-off-by: Rob Jones <rob.jones@codethink.co.uk>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>