dma-buf: fix check in dma_resv_add_fence

It's valid to add the same fence multiple times to a dma-resv object and
we shouldn't need one extra slot for each.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: a3f7c10a ("dma-buf/dma-resv: check if the new fence is really later")
Cc: stable@vger.kernel.org # v5.19+
Link: https://patchwork.freedesktop.org/patch/msgid/20231115093035.1889-1-christian.koenig@amd.com