netfilter: nf_tables: de-constify set commit ops function argument

[ Upstream commit 25600167 ]

The set backend using this already has to work around this via ugly
cast, don't spread this pattern.

Signed-off-by: Florian Westphal <fw@strlen.de>
Stable-dep-of: 8d738c18 ("netfilter: nf_tables: fix set size with rbtree backend")
Signed-off-by: Sasha Levin <sashal@kernel.org>