netfilter: make function op structures const

[ Upstream commit 285c8a7a

 ]

No functional changes, these structures should be const.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 62e7151a

 ("netfilter: bridge: confirm multicast packets before passing them up the stack")
Signed-off-by: Sasha Levin <sashal@kernel.org>