netfilter: Remove checks of seq_printf() return values

The return value of seq_printf() is soon to be removed. Remove the
checks from seq_printf() in favor of seq_has_overflowed().

Link: http://lkml.kernel.org/r/20141104142236.GA10239@salvia
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Cc: netfilter-devel@vger.kernel.org
Cc: coreteam@netfilter.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>