mpls: Use in6_dev_rcu() and dev_net_rcu() in mpls_forward() and mpls_xmit().

mpls: Use in6_dev_rcu() and dev_net_rcu() in mpls_forward() and mpls_xmit().

mpls_forward() and mpls_xmit() are called under RCU.

Let's use in6_dev_rcu() and dev_net_rcu() there to annotate
as such.

Now we pass net to mpls_stats_inc_outucastpkts() not to read
dev_net_rcu() twice.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20251029173344.2934622-6-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>