mpls: Drop RTNL for RTM_NEWROUTE, RTM_DELROUTE, and RTM_GETROUTE.

mpls: Drop RTNL for RTM_NEWROUTE, RTM_DELROUTE, and RTM_GETROUTE.

RTM_NEWROUTE looks up dev under RCU (ip_route_output(),
ipv6_stub->ipv6_dst_lookup_flow(), netdev_get_by_index()),
and each neighbour holds the refcnt of its dev.

Also, net->mpls.platform_label is protected by a dedicated
per-netns mutex.

Now, no MPLS code depends on RTNL.

Let's drop RTNL for RTM_NEWROUTE, RTM_DELROUTE, and RTM_GETROUTE.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20251029173344.2934622-14-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>