dma-buf: actually set signaling bit for private stub fences

dma-buf: actually set signaling bit for private stub fences

In dma_fence_allocate_private_stub() set the signaling bit of the newly
allocated private stub fence rather than the signaling bit of the
shared dma_fence_stub.

Fixes: c85d00d4fd8b ("dma-buf: set signaling bit for the stub fence")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230126002844.339593-1-dakr@redhat.com
(cherry picked from commit 851a4a77a9f6441bd73625fe6dbc29c814ae681f)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>