drm/syncobj: Don't leak fences when WAIT_FOR_SUBMIT is set

We attempt to get fences earlier in the hopes that everything will
already have fences and no callbacks will be needed. If we do succeed
in getting a fence, getting one a second time will result in a duplicate
ref with no unref. This is causing memory leaks in Vulkan applications
that create a lot of fences; playing for a few hours can, apparently,
bring down the system.

Cc: [e-mail]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107899
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Jason Ekstrand <[e-mail]>
Signed-off-by: Sean Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_syncobj.c

···
 {
 	int ret;
 
+	WARN_ON(*fence);
+
 	*fence = drm_syncobj_fence_get(syncobj);
 	if (*fence)
 		return 1;
···
 
 	if (flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT) {
 		for (i = 0; i < count; ++i) {
+			if (entries[i].fence)
+				continue;
+
 			drm_syncobj_fence_get_or_add_callback(syncobjs[i],
 							      &entries[i].fence,
 							      &entries[i].syncobj_cb,