ipmi: msghandler: Suppress suspicious RCU usage warning

while running ipmi, ipmi_smi_watcher_register() caused
a suspicious RCU usage warning.

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=============================
WARNING: suspicious RCU usage
5.10.0-rc3+ #1 Not tainted
-----------------------------
drivers/char/ipmi/ipmi_msghandler.c:750 RCU-list traversed in non-reader section!!
other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1
2 locks held by syz-executor.0/4254:
stack backtrace:
CPU: 0 PID: 4254 Comm: syz-executor.0 Not tainted 5.10.0-rc3+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/ 01/2014
Call Trace:
dump_stack+0x19d/0x200
ipmi_smi_watcher_register+0x2d3/0x340 [ipmi_msghandler]
acpi_ipmi_init+0xb1/0x1000 [acpi_ipmi]
do_one_initcall+0x149/0x7e0
do_init_module+0x1ef/0x700
load_module+0x3467/0x4140
__do_sys_finit_module+0x10d/0x1a0
do_syscall_64+0x34/0x80
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x468ded

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It is safe because smi_watchers_mutex is locked and srcu_read_lock
has been used, so simply pass lockdep_is_held() to the
list_for_each_entry_rcu() to suppress this warning.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Message-Id: <20201119070839.381-1-miaoqinglang@huawei.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

drivers/char/ipmi/ipmi_msghandler.c

@@ -747,7 +747,8 @@ int ipmi_smi_watcher_register(struct ipmi_smi_watcher *watcher)
 	list_add(&watcher->link, &smi_watchers);
 
 	index = srcu_read_lock(&ipmi_interfaces_srcu);
-	list_for_each_entry_rcu(intf, &ipmi_interfaces, link) {
+	list_for_each_entry_rcu(intf, &ipmi_interfaces, link,
+			lockdep_is_held(&smi_watchers_mutex)) {
 		int intf_num = READ_ONCE(intf->intf_num);
 
 		if (intf_num == -1)