dma-debug: reorder struct dma_debug_entry fields

Move all fields used during exact match lookups to the first cache line.
This makes debug_dma_mapping_error() and friends about 50% faster.

Since it removes two 32bit holes, force a cacheline alignment on struct
dma_debug_entry.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>