drm/amdkfd: Work around mmu_notifier_put issue on RHEL 8.3

The DRM backport from kernel 5.6 includes some MMU notifier changes
that cause problems with the mmu_notifier_put function. The
free_notifier never gets called. This leads to a leak of kfd_process
structures and their doorbells.

Work around this by falling back to the old method of releasing the
MMU notifier and destryoing the process structure.

Change-Id: I711a61efee2b284e385251b7f826e228e64c04f2
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Flora Cui<flora.cui@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_priv.h

@@ -864,6 +864,11 @@ struct kfd_process {
 	/* We want to receive a notification when the mm_struct is destroyed */
 	struct mmu_notifier mmu_notifier;
 
+#if defined(DRM_VER) && defined(DRM_PATCH) && DRM_VER == 5 && DRM_PATCH == 6 \
+	&& LINUX_VERSION_CODE == KERNEL_VERSION(4, 18, 0)
+	/* mmu_notifier_put in the RH DRM backport from 5.6 is broken */
+#	undef HAVE_MMU_NOTIFIER_PUT
+#endif
 #ifndef HAVE_MMU_NOTIFIER_PUT
 	/* Use for delayed freeing of kfd_process structure */
 	struct rcu_head	rcu;