bridge: Fix sleep in atomic context in netlink path

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

bridge: Fix sleep in atomic context in netlink path

Since the introduction of the netlink configuration path for bridge
ports in commit 25c71c75ac87 ("bridge: bridge port parameters over
netlink"), br_setport() was always called with the bridge lock held
around it. Back then this decision made sense: The bridge lock protects
the STP state of the bridge and its ports and at that time the function
only processed three STP related netlink attributes (cost, priority and
state).

Nowadays, br_setport() processes a lot more attributes and most of them
do not need the bridge lock:

* Bridge flags: Only require RTNL. Read locklessly by the data path.
Annotations can be added in net-next.

* FDB port flushing: Only requires the FDB lock.

* Multicast attributes: Only require the multicast lock.

* Group forward mask: Only requires RTNL. Read locklessly by the data
path. Annotations can be added in net-next.

* Backup port and NHID: Only require RTNL. Read locklessly by the data
path.

This is a problem as the bridge calls dev_set_promiscuity() when certain
bridge port flags change and this function can sleep since the commit
cited below, resulting in a splat such as [1].

Fix this by reducing the scope of the bridge lock and only take it when
processing the three STP related attributes that require it. This is
consistent with the multicast attributes where each attribute acquires
the multicast lock instead of having one critical section for all
relevant attributes.

[1]
BUG: sleeping function called from invalid context at net/core/dev_addr_lists.c:1262
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 356, name: bridge
preempt_count: 201, expected: 0
RCU nest depth: 0, expected: 0
2 locks held by bridge/356:
#0: ffffffff919473a0 (rtnl_mutex){+.+.}-{4:4}, at: rtnetlink_rcv_msg (net/core/rtnetlink.c:80 net/core/rtnetlink.c:7002)
#1: ffff888115072d58 (&br->lock){+...}-{3:3}, at: br_setlink (./include/linux/spinlock.h:348 net/bridge/br_netlink.c:1117)
Preemption disabled at:
0x0
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Call Trace:
<TASK>
dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
__might_resched.cold (kernel/sched/core.c:9163)
netif_rx_mode_run (net/core/dev_addr_lists.c:1262)
netif_rx_mode_sync (net/core/dev_addr_lists.c:1428)
dev_set_promiscuity (net/core/dev_api.c:289)
br_manage_promisc (net/bridge/br_if.c:135 net/bridge/br_if.c:172)
br_port_flags_change (net/bridge/br_if.c:242 net/bridge/br_if.c:747)
br_setport (net/bridge/br_netlink.c:1000)
br_setlink (net/bridge/br_netlink.c:1118)
rtnl_bridge_setlink (net/core/rtnetlink.c:5572)
rtnetlink_rcv_msg (net/core/rtnetlink.c:7005)
netlink_rcv_skb (net/netlink/af_netlink.c:2550)
netlink_unicast (net/netlink/af_netlink.c:1318 net/netlink/af_netlink.c:1344)
netlink_sendmsg (net/netlink/af_netlink.c:1894)
__sock_sendmsg (net/socket.c:787 (discriminator 4) net/socket.c:802 (discriminator 4))
____sys_sendmsg (net/socket.c:2698)
___sys_sendmsg (net/socket.c:2752)
__sys_sendmsg (net/socket.c:2784)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)