xfrm: add missing RCU read lock in xfrm_send_migrate_state()

xfrm_nlmsg_multicast() requires the RCU read lock to be held because it
safely dereferences the net->xfrm.nlsk pointer using rcu_dereference().
When it is called from xfrm_send_migrate_state(), the RCU read lock is not
held, which triggers a suspicious RCU usage warning:

WARNING: suspicious RCU usage
net/xfrm/xfrm_user.c:1630 suspicious rcu_dereference_check() usage!

Call Trace:
 lockdep_rcu_suspicious+0x13f/0x1d0 kernel/locking/lockdep.c:6876
 xfrm_nlmsg_multicast+0x1d8/0x1f0 net/xfrm/xfrm_user.c:1630
 xfrm_send_migrate_state+0x870/0xae0 net/xfrm/xfrm_user.c:3340
 xfrm_do_migrate_state+0x1749/0x1e90 net/xfrm/xfrm_user.c:3507
 xfrm_user_rcv_msg+0x7a8/0xf30 net/xfrm/xfrm_user.c:3907

Fix this by wrapping the xfrm_nlmsg_multicast() call in
xfrm_send_migrate_state() with rcu_read_lock() and rcu_read_unlock().

Fixes: a9d155ea9b44 ("xfrm: add XFRM_MSG_MIGRATE_STATE for single SA migration")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+c0e99a1aa85a286d7a3b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c0e99a1aa85a286d7a3b
Link: https://syzkaller.appspot.com/ai_job?id=8977f559-3a7e-4bb5-b4d6-1196956260b6
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>