netfilter: delete repeated words

Drop duplicated words in net/netfilter/ and net/ipv4/netfilter/.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>