route_dumpit() already relies on RCU, RTNL is not needed.

Also change return value at the end of a dump.
This allows NLMSG_DONE to be appended to the current
skb at the end of a dump, saving a couple of recvmsg()
system calls.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Remi Denis-Courmont <courmisch@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240507121748.416287-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>