netfilter: xt_recent: recent_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 the patch:
 # dd if=/proc/net/xt_recent/SSH # original file outpt
 src=127.0.0.4 ttl: 0 last_seen: 6275444819 oldest_pkt: 1 6275444819
 src=127.0.0.2 ttl: 0 last_seen: 6275438906 oldest_pkt: 1 6275438906
 src=127.0.0.3 ttl: 0 last_seen: 6275441953 oldest_pkt: 1 6275441953
 0+1 records in
 0+1 records out
 204 bytes copied, 6.1332e-05 s, 3.3 MB/s

Read after lseek into middle of last line (offset 140 in example below)
generates expected end of last line and then unexpected whole last line
once again

 # dd if=/proc/net/xt_recent/SSH bs=140 skip=1
 dd: /proc/net/xt_recent/SSH: cannot skip to specified offset
 127.0.0.3 ttl: 0 last_seen: 6275441953 oldest_pkt: 1 6275441953
 src=127.0.0.3 ttl: 0 last_seen: 6275441953 oldest_pkt: 1 6275441953
 0+1 records in
 0+1 records out
 132 bytes copied, 6.2487e-05 s, 2.1 MB/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>