dma-buf/sync_file: Don't leak fences on merge failure

In the Linux kernel, the following vulnerability has been resolved:

dma-buf/sync_file: Don't leak fences on merge failure

Each add_fence() call does a dma_fence_get() on the relevant fence. In

the error path, we weren't calling dma_fence_put() so all those fences

got leaked. Also, in the krealloc_array failure case, we weren't

freeing the fences array. Instead, ensure that i and fences are always

zero-initialized and dma_fence_put() all the fences and kfree(fences) on

every error path.