dma-buf/dma-resv: Stop leaking on krealloc() failure

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

dma-buf/dma-resv: Stop leaking on krealloc() failure

Currently dma_resv_get_fences() will leak the previously allocated array if the fence iteration got restarted and the krealloc_array() fails.

Free the old array by hand, and make sure we still clear the returned *fences so the caller won't end up accessing freed memory. Some (but not all) of the callers of dma_resv_get_fences() seem to still trawl through the array even when dma_resv_get_fences() failed. And let's zero out *num_fences as well for good measure.