locking/ww_mutex: Turn off __must_check for now

With the ww_mutex inline wrappers gone there's a lot of dormant
anti-patterns emerging in an x86 allyesconfig build:

  kernel/locking/test-ww_mutex.c:80:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  kernel/locking/test-ww_mutex.c:55:3: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  kernel/locking/test-ww_mutex.c:134:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  kernel/locking/test-ww_mutex.c:213:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  kernel/locking/test-ww_mutex.c:177:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  kernel/locking/test-ww_mutex.c:266:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:213:19: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  lib/locking-selftest.c:211:20: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/drm_modeset_lock.c:430:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c:70:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/vgem/vgem_fence.c:193:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/i915/i915_gem_batch_pool.c:125:4: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/i915/i915_gem_execbuffer.c:1302:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/radeon/radeon_prime.c:69:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]
  drivers/gpu/drm/nouveau/nouveau_prime.c:70:2: warning: ignoring return value of ‘ww_mutex_lock’, declared with attribute warn_unused_result [-Wunused-result]

... but we cannot just litter the kernel build log with such warnings.

These need to be fixed separately - turn off the warning for now.

Cc: Nicolai Hähnle <nicolai.haehnle@amd.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/ww_mutex.h

@@ -215,8 +215,7 @@ static inline void ww_acquire_fini(struct ww_acquire_ctx *ctx)
  *
  * A mutex acquired with this function must be released with ww_mutex_unlock.
  */
-extern int __must_check ww_mutex_lock(struct ww_mutex *lock,
-				      struct ww_acquire_ctx *ctx);
+extern int /* __must_check */ ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx);
 
 /**
  * ww_mutex_lock_interruptible - acquire the w/w mutex, interruptible