Fix dbuf_stats_hash_table_data race

Dropping DBUF_HASH_MUTEX when walking the hash list is unsafe. The dbuf
can be freed at any time.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4846