neighbour: use RCU protection in __neigh_notify()

neighbour: use RCU protection in __neigh_notify()

__neigh_notify() can be called without RTNL or RCU protection.

Use RCU protection to avoid potential UAF.

Fixes: 426b5303eb43 ("[NETNS]: Modify the neighbour table code so it handles multiple network namespaces")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250207135841.1948589-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>