netfilter: Use correct return for seq_show functions

Using seq_has_overflowed doesn't produce the right return value.
Either 0 or -1 is, but 0 is much more common and works well when
seq allocation retries.

I believe this doesn't matter as the initial allocation is always
sufficient, this is just a correctness patch.

Miscellanea:

o Don't use strlen, use *ptr to determine if a string
  should be emitted like all the other tests here
o Delete unnecessary return statements

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>