dma-buf/resv: fix exclusive fence get

This causes kernel crash when testing lima driver.

Cc: Christian König <christian.koenig@amd.com>
Fixes: b8c036df ("dma-buf: simplify reservation_object_get_fences_rcu a bit")
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190922074900.853-1-yuq825@gmail.com

drivers/dma-buf/dma-resv.c

@@ -471,7 +471,7 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 	if (pfence_excl)
 		*pfence_excl = fence_excl;
 	else if (fence_excl)
-		shared[++shared_count] = fence_excl;
+		shared[shared_count++] = fence_excl;
 
 	if (!shared_count) {
 		kfree(shared);