xfrm: add missing rcu_read_lock(), skb_dst_force() and dev_hold() for xfrm_trans_reinject()

syzbot reported a suspicious RCU usage warning in ip6_pkt_drop():

  WARNING: suspicious RCU usage in ip6_pkt_drop
  include/net/addrconf.h:389 suspicious rcu_dereference_check() usage!

  Call Trace:
   __in6_dev_get_safely include/net/addrconf.h:389 [inline]
   ip6_pkt_drop+0x596/0x610 net/ipv6/route.c:4620
   ip6_pkt_discard+0x1c/0x30 net/ipv6/route.c:4651
   xfrm_trans_reinject+0x324/0x630 net/xfrm/xfrm_input.c:806
   process_one_work kernel/workqueue.c:3322 [inline]
   process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
   worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486

When commit 4f4920669d21 ("xfrm: Reinject transport-mode packets through
workqueue") converted xfrm_trans_reinject from a tasklet to a workqueue,
the reinjection loop ceased running in softirq context. Workqueue workers
run in process context where local_bh_disable() does not enter an RCU
read-side critical section under CONFIG_PREEMPT_RCU.

Because finish callbacks (such as ip6_rcv_finish) expect to run under an
RCU read lock (performing route lookups, l3mdev lookups, and accessing
RCU-protected data structures), invoking them in workqueue context without
rcu_read_lock() triggers RCU lockdep warnings.

Furthermore, packets queued to the workqueue via xfrm_trans_queue_net()
may carry non-refcounted (noref) dst entries (e.g. from ip_route_input_noref).
Additionally, on netdevice unregistration, dst_dev_put() replaces dst->dev
with blackhole_netdev, so dst entries do not keep skb->dev alive while
queued in the workqueue.

Fix these issues by:
1. Calling skb_dst_force(skb) in xfrm_trans_queue_net() while still in the
   caller's RCU section to ensure dst is reference-counted before queuing.
2. Holding a reference on skb->dev via dev_hold()/dev_put() across workqueue
   deferral so skb->dev remains valid during finish() callback processing.
3. Acquiring rcu_read_lock() around the finish callback invocation loop in
   xfrm_trans_reinject().

Fixes: 4f4920669d21 ("xfrm: Reinject transport-mode packets through workqueue")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Liu Jian <liujian56@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>