i2c-core: Use per-adapter userspace device lists

Using a single list for all userspace devices leads to a dead lock
on multiplexed buses in some circumstances (mux chip instantiated
from userspace). This is solved by using a separate list for each
bus segment.

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

include/linux/i2c.h

@@ -355,6 +355,8 @@ struct i2c_adapter {
 	int nr;
 	char name[48];
 	struct completion dev_released;
+
+	struct list_head userspace_clients;
 };
 #define to_i2c_adapter(d) container_of(d, struct i2c_adapter, dev)