af_packet: move notifier's packet_dev_mc out of rcu critical section

In the Linux kernel, the following vulnerability has been resolved:

af_packet: move notifier's packet_dev_mc out of rcu critical section

Syzkaller reports the following issue:

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:578
__mutex_lock+0x106/0xe80 kernel/locking/mutex.c:746
team_change_rx_flags+0x38/0x220 drivers/net/team/team_core.c:1781
dev_change_rx_flags net/core/dev.c:9145 [inline]
__dev_set_promiscuity+0x3f8/0x590 net/core/dev.c:9189
netif_set_promiscuity+0x50/0xe0 net/core/dev.c:9201
dev_set_promiscuity+0x126/0x260 net/core/dev_api.c:286 packet_dev_mc net/packet/af_packet.c:3698 [inline]
packet_dev_mclist_delete net/packet/af_packet.c:3722 [inline]
packet_notifier+0x292/0xa60 net/packet/af_packet.c:4247
notifier_call_chain+0x1b3/0x3e0 kernel/notifier.c:85
call_netdevice_notifiers_extack net/core/dev.c:2214 [inline]
call_netdevice_notifiers net/core/dev.c:2228 [inline]
unregister_netdevice_many_notify+0x15d8/0x2330 net/core/dev.c:11972
rtnl_delete_link net/core/rtnetlink.c:3522 [inline]
rtnl_dellink+0x488/0x710 net/core/rtnetlink.c:3564
rtnetlink_rcv_msg+0x7cf/0xb70 net/core/rtnetlink.c:6955
netlink_rcv_skb+0x219/0x490 net/netlink/af_netlink.c:2534

Calling `PACKET_ADD_MEMBERSHIP` on an ops-locked device can trigger
the `NETDEV_UNREGISTER` notifier, which may require disabling promiscuous
and/or allmulti mode. Both of these operations require acquiring
the netdev instance lock.

Move the call to `packet_dev_mc` outside of the RCU critical section.
The `mclist` modifications (add, del, flush, unregister) are protected by
the RTNL, not the RCU. The RCU only protects the `sklist` and its
associated `sks`. The delayed operation on the `mclist` entry remains
within the RTNL.