netfilter: nf_tables: place base_seq in struct net

[ Upstream commit 64102d9b ]

This will soon be read from packet path around same time as the gencursor.

Both gencursor and base_seq get incremented almost at the same time, so
it makes sense to place them in the same structure.

This doesn't increase struct net size on 64bit due to padding.

Signed-off-by: Florian Westphal <fw@strlen.de>
Stable-dep-of: b2f742c8 ("netfilter: nf_tables: restart set lookup on base_seq change")
Signed-off-by: Sasha Levin <sashal@kernel.org>