net: add rcu safety to rtnl_prop_list_size()

rtnl_prop_list_size() can be called while alternative names
are added or removed concurrently.

if_nlmsg_size() / rtnl_calcit() can indeed be called
without RTNL held.

Use explicit RCU protection to avoid UAF.

Fixes: 88f4fb0c

 ("net: rtnetlink: put alternative names to getlink message")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240209181248.96637-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>