igbvf: Fix leak in TX DMA error cleanup

In the Linux kernel, the following vulnerability has been resolved: igbvf: Fix leak in TX DMA error cleanup If an error is encountered while mapping TX buffers, the driver should unmap any buffers already mapped for that skb. Because count is incremented before each frag mapping, it will always match the correct number of unmappings needed when dma_error is reached. Decrementing count before the while loop in dma_error causes an off-by-one error. If any mapping was successful before an unsuccessful mapping, exactly one DMA mapping (the head) would leak. This bug was introduced by a 2010 fix for an endless loop in dma_error. All other affected drivers have already been fixed.