FreeBSD: Update dataset_kstats for zvols in dev mode

Previous commit added accounting for geom mode, but not for dev.
In geom mode we actually have GEOM statistics, while in dev mode
additional accounting actually makes more sense.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes openzfs#12097

module/os/freebsd/zfs/zvol_os.c

@@ -856,6 +856,7 @@ zvol_cdev_read(struct cdev *dev, struct uio *uio_s, int ioflag)
 	    (zfs_uio_offset(&uio) < 0 || zfs_uio_offset(&uio) > volsize))
 		return (SET_ERROR(EIO));
 
+	ssize_t start_resid = zfs_uio_resid(&uio);
 	lr = zfs_rangelock_enter(&zv->zv_rangelock, zfs_uio_offset(&uio),
 	    zfs_uio_resid(&uio), RL_READER);
 	while (zfs_uio_resid(&uio) > 0 && zfs_uio_offset(&uio) < volsize) {
@@ -874,6 +875,8 @@ zvol_cdev_read(struct cdev *dev, struct uio *uio_s, int ioflag)
 		}
 	}
 	zfs_rangelock_exit(lr);
+	int64_t nread = start_resid - zfs_uio_resid(&uio);
+	dataset_kstats_update_read_kstats(&zv->zv_kstat, nread);
 
 	return (error);
 }
@@ -898,6 +901,7 @@ zvol_cdev_write(struct cdev *dev, struct uio *uio_s, int ioflag)
 	    (zfs_uio_offset(&uio) < 0 || zfs_uio_offset(&uio) > volsize))
 		return (SET_ERROR(EIO));
 
+	ssize_t start_resid = zfs_uio_resid(&uio);
 	sync = (ioflag & IO_SYNC) ||
 	    (zv->zv_objset->os_sync == ZFS_SYNC_ALWAYS);
 
@@ -929,6 +933,8 @@ zvol_cdev_write(struct cdev *dev, struct uio *uio_s, int ioflag)
 			break;
 	}
 	zfs_rangelock_exit(lr);
+	int64_t nwritten = start_resid - zfs_uio_resid(&uio);
+	dataset_kstats_update_write_kstats(&zv->zv_kstat, nwritten);
 	if (sync)
 		zil_commit(zv->zv_zilog, ZVOL_OBJ);
 	rw_exit(&zv->zv_suspend_lock);