dma-debug: increase HASH_SIZE

With modern NIC, it is not unusual having about ~256,000 active dma
mappings and a hash size of 1024 buckets is too small.

Forcing full cache line per bucket does not seem useful, especially now
that we have contention on free_entries_lock for allocations and freeing
of entries.  Better use the space to fit more buckets.

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