Add snapshots_changed_nsecs dataset property

Add a read-only dataset property, snapshots_changed_nsecs, which 
exposes the nanosecond resolution version of snapshots_changed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Wolfgang Hoschek <wolfgang.hoschek@mac.com>
Closes #17998
Closes #18031

include/sys/fs/zfs.h

@@ -203,6 +203,7 @@ typedef enum {
 	ZFS_PROP_DEFAULTUSEROBJQUOTA,
 	ZFS_PROP_DEFAULTGROUPOBJQUOTA,
 	ZFS_PROP_DEFAULTPROJECTOBJQUOTA,
+	ZFS_PROP_SNAPSHOTS_CHANGED_NSECS,
 	ZFS_NUM_PROPS
 } zfs_prop_t;