netfilter: fix a few (harmless) sparse warnings

net/netfilter/nft_payload.c:187:18: warning: incorrect type in return expression (expected bool got restricted __sum16 [usertype] check)
net/netfilter/nft_exthdr.c:222:14: warning: cast to restricted __be32
net/netfilter/nft_rt.c:49:23: warning: incorrect type in assignment (different base types expected unsigned int got restricted __be32)
net/netfilter/nft_rt.c:70:25: warning: symbol 'nft_rt_policy' was not declared. Should it be static?

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