DDT: Switch to using wmsums for lookup stats

ddt_lookup() is a very busy code under a highly congested global
lock. Anything we can save here is very important.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17980