Expand the FreeBSD spl for kstats to support all current types
Move the dataset_kstats_t back to zvol_state_t from zfs_state_os_t
now that it is common once again
```
kstat.zfs/mypool.dataset.objset-0x10b.nunlinked: 0
kstat.zfs/mypool.dataset.objset-0x10b.nunlinks: 0
kstat.zfs/mypool.dataset.objset-0x10b.nread: 150528
kstat.zfs/mypool.dataset.objset-0x10b.reads: 48
kstat.zfs/mypool.dataset.objset-0x10b.nwritten: 134217728
kstat.zfs/mypool.dataset.objset-0x10b.writes: 1024
kstat.zfs/mypool.dataset.objset-0x10b.dataset_name: mypool/datasetname
```
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes#10386
4547fc4e07