Update naming of per-dataset counters introduced in r365689

Upstream OpenZFS is changing the structure of the sysctl mibs to avoid
having a / in the element name. Chase this change here to avoid introducing
a new sysctl that will change named between 12.2 and 13.0

MFC-with: 365689
Relnotes: yes (fix example)
Sponsored by: Klara Inc.