i2c: Use a separate mutex for userspace client lists

Moving userspace-instantiated clients to separate lists wasn't nearly
enough to avoid deadlocks in multiplexed bus cases. We also want to
have a dedicated mutex to protect each list.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Michael Lawnick <ml.lawnick@gmx.de>

include/linux/i2c.h

@@ -368,6 +368,7 @@ struct i2c_adapter {
 	char name[48];
 	struct completion dev_released;
 
+	struct mutex userspace_clients_lock;
 	struct list_head userspace_clients;
 };
 #define to_i2c_adapter(d) container_of(d, struct i2c_adapter, dev)