Switch dnode stats to wmsums

I've noticed that some of those counters are used in hot paths like
dnode_hold_impl(), and results of this change is visible in profiler.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #14198