phonet: Convert phonet_device_list.lock to spinlock_t.

addr_doit() calls phonet_address_add() or phonet_address_del()
for RTM_NEWADDR or RTM_DELADDR, respectively.

Both functions only touch phonet_device_list(dev_net(dev)),
which is currently protected by RTNL and its dedicated mutex,
phonet_device_list.lock.

We will convert addr_doit() to RCU and cannot use mutex inside RCU.

Let's convert the mutex to spinlock_t.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/phonet/pn_dev.h

@@ -12,12 +12,13 @@
 
 #include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/spinlock.h>
 
 struct net;
 
 struct phonet_device_list {
 	struct list_head list;
-	struct mutex lock;
+	spinlock_t lock;
 };
 
 struct phonet_device_list *phonet_device_list(struct net *net);