mpls: Add mpls_route_input().

mpls: Add mpls_route_input().

mpls_route_input_rcu() is called from mpls_forward() and
mpls_getroute().

The former is under RCU, and the latter is under RTNL, so
mpls_route_input_rcu() uses rcu_dereference_rtnl().

Let's use rcu_dereference() in mpls_route_input_rcu() and
add an RTNL variant for mpls_getroute().

Later, we will remove rtnl_dereference() there.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20251029173344.2934622-9-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>