drm: fix drmm_mutex_init()

In mutex_init() lockdep identifies a lock by defining a special static
key for each lock class. However if we wrap the macro in a function,
like in drmm_mutex_init(), we end up generating:

int drmm_mutex_init(struct drm_device *dev, struct mutex *lock)
{
      static struct lock_class_key __key;

      __mutex_init((lock), "lock", &__key);
      ....
}

The static __key here is what lockdep uses to identify the lock class,
however since this is just a normal function the key here will be
created once, where all callers then use the same key. In effect the
mutex->depmap.key will be the same pointer for different
drmm_mutex_init() callers. This then results in impossible lockdep
splats since lockdep thinks completely unrelated locks are the same lock
class.

To fix this turn drmm_mutex_init() into a macro such that it generates a
different "static struct lock_class_key __key" for each invocation,
which looks to be inline with what mutex_init() wants.

v2:
  - Revamp the commit message with clearer explanation of the issue.
  - Rather export __drmm_mutex_release() than static inline.

Reported-by: Thomas Hellström <[email protected]>
Reported-by: Sarah Walker <[email protected]>
Fixes: e13f13e039dc ("drm: Add DRM-managed mutex_init()")
Cc: Stanislaw Gruszka <[email protected]>
Cc: Boris Brezillon <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Jocelyn Falempe <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Signed-off-by: Matthew Auld <[email protected]>
Reviewed-by: Boris Brezillon <[email protected]>
Reviewed-by: Stanislaw Gruszka <[email protected]>
Reviewed-by: Lucas De Marchi <[email protected]>
Acked-by: Thomas Zimmermann <[email protected]>
Signed-off-by: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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