netfilter: Use seq_puts()/seq_putc() where possible

For string without format specifiers, use seq_puts(). For
seq_printf("\n"), use seq_putc('\n').

Signed-off-by: simran singhal <singhalsimran0@gmail.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>