misc: open-dice: Fix spurious lockdep warning

When probing the open-dice driver with PROVE_LOCKING=y, lockdep
complains that the mutex in 'drvdata->lock' has a non-static key:

 | INFO: trying to register non-static key.
 | The code is fine but needs lockdep annotation, or maybe
 | you didn't initialize this object before use?
 | turning off the locking correctness validator.

Fix the problem by initialising the mutex memory with mutex_init()
instead of __MUTEX_INITIALIZER().

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Brazdil <dbrazdil@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240126152410.10148-1-will@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: willdeacon

drivers/misc/open-dice.c

@@ -140,7 +140,6 @@ static int __init open_dice_probe(struct platform_device *pdev)
 		return -ENOMEM;
 
 	*drvdata = (struct open_dice_drvdata){
-		.lock = __MUTEX_INITIALIZER(drvdata->lock),
 		.rmem = rmem,
 		.misc = (struct miscdevice){
 			.parent	= dev,
@@ -150,6 +149,7 @@ static int __init open_dice_probe(struct platform_device *pdev)
 			.mode	= 0600,
 		},
 	};
+	mutex_init(&drvdata->lock);
 
 	/* Index overflow check not needed, misc_register() will fail. */
 	snprintf(drvdata->name, sizeof(drvdata->name), DRIVER_NAME"%u", dev_idx++);