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.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>