Add snapshots_changed as property

Make dd_snap_cmtime property persistent across mount and unmount
operations by storing in ZAP and restore the value from ZAP on hold
into dd_snap_cmtime instead of updating it.

Expose dd_snap_cmtime as 'snapshots_changed' property that provides a
mechanism to quickly determine whether snapshot list for dataset has
changed without having to mount a dataset or iterate the snapshot list.

It specifies the time at which a snapshot for a dataset was last
created or deleted. This allows us to be more efficient how often we
query snapshots.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13635

include/sys/fs/zfs.h

@@ -189,6 +189,7 @@ typedef enum {
 	ZFS_PROP_IVSET_GUID,		/* not exposed to the user */
 	ZFS_PROP_REDACTED,
 	ZFS_PROP_REDACT_SNAPS,
+	ZFS_PROP_SNAPSHOTS_CHANGED,
 	ZFS_NUM_PROPS
 } zfs_prop_t;