neighbour: fix various data-races

neighbour: fix various data-races

[ Upstream commit a9beb7e81bcb876615e1fbb3c07f3f9dba69831f ]

1) tbl->gc_thresh1, tbl->gc_thresh2, tbl->gc_thresh3 and tbl->gc_interval
   can be written from sysfs.

2) tbl->last_flush is read locklessly from neigh_alloc()

3) tbl->proxy_queue.qlen is read locklessly from neightbl_fill_info()

4) neightbl_fill_info() reads cpu stats that can be changed concurrently.

Fixes: c7fb64db001f ("[NETLINK]: Neighbour table configuration and statistics via rtnetlink")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20231019122104.1448310-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>