drm/vc4: Fix false positive WARN() backtrace on refcount_inc() usage

With CONFIG_REFCOUNT_FULL enabled, refcount_inc() complains when it's
passed a refcount object that has its counter set to 0. In this driver,
this is a valid use case since we want to increment ->usecnt only when
the BO object starts to be used by real HW components and this is
definitely not the case when the BO is created.

Fix the problem by using refcount_inc_not_zero() instead of
refcount_inc() and fallback to refcount_set(1) when
refcount_inc_not_zero() returns false. Note that this 2-steps operation
is not racy here because the whole section is protected by a mutex
which guarantees that the counter does not change between the
refcount_inc_not_zero() and refcount_set() calls.

Fixes: b9f19259 ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20171122203928.28135-1-boris.brezillon@free-electrons.com

drivers/gpu/drm/vc4/vc4_bo.c

@@ -637,7 +637,8 @@ int vc4_bo_inc_usecnt(struct vc4_bo *bo)
 	mutex_lock(&bo->madv_lock);
 	switch (bo->madv) {
 	case VC4_MADV_WILLNEED:
-		refcount_inc(&bo->usecnt);
+		if (!refcount_inc_not_zero(&bo->usecnt))
+			refcount_set(&bo->usecnt, 1);
 		ret = 0;
 		break;
 	case VC4_MADV_DONTNEED: