PROBLEM: (91352) (PATCH ID: OSF520-550)
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This problem would only be seen under high utilization of DMA mapping resources,
and typically would affect only certain device drivers. The device driver would
have had to be doing an "alloc on-the-fly" (i.e., did a dma_map_load() without
a prior dma_map_alloc()). In the rare case that dma_map_load returned 0,
indicating failure, the dma_handle returned to the caller would point to 
previously malloc'd memory that by this point has been freed. This dma_handle
is stale. 
This patch ensures that upon dma_map_load() failure conditions, the dma_handle
that is returned is NULL.