netfilter: Remove uses of seq_<foo> return values

The seq_printf/seq_puts/seq_putc return values, because they
are frequently misused, will eventually be converted to void.

See: commit 1f33c41c03da ("seq_file: Rename seq_overflow() to
     seq_has_overflowed() and make public")

Miscellanea:

o realign arguments

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