netfilter: x_tables: xt_mttg_seq_next should increase position index

If .next function does not change position index,
following .show function will repeat output related
to current position index.

Without patch:
 # dd if=/proc/net/ip_tables_matches  # original file output
 conntrack
 conntrack
 conntrack
 recent
 recent
 icmp
 udplite
 udp
 tcp
 0+1 records in
 0+1 records out
 65 bytes copied, 5.4074e-05 s, 1.2 MB/s

 # dd if=/proc/net/ip_tables_matches bs=62 skip=1
 dd: /proc/net/ip_tables_matches: cannot skip to specified offset
 cp   <<< end of  last line
 tcp  <<< and then unexpected whole last line once again
 0+1 records in
 0+1 records out
 7 bytes copied, 0.000102447 s, 68.3 kB/s

Cc: stable@vger.kernel.org
Fixes: 1f4aace6

 ("fs/seq_file.c: simplify seq_file iteration code ...")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206283
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>