Updates for snapshots_changed property

Currently, snapshots_changed property is stored in dd_props_zapobj, due
to which the property is assumed to be local. This causes a difference
in behavior with respect to other readonly properties.

This commit stores the snapshots_changed property in dd_object. Source
is not set to local in this case, which makes it consistent with other
readonly properties.

This commit also updates the date string format to include seconds.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13785

lib/libzfs/libzfs_dataset.c

@@ -2947,7 +2947,7 @@ zfs_prop_get(zfs_handle_t *zhp, zfs_prop_t prop, char *propbuf, size_t proplen,
 
 			if (literal ||
 			    localtime_r(&time, &t) == NULL ||
-			    strftime(propbuf, proplen, "%a %b %e %k:%M %Y",
+			    strftime(propbuf, proplen, "%a %b %e %k:%M:%S %Y",
 			    &t) == 0)
 				(void) snprintf(propbuf, proplen, "%llu",
 				    (u_longlong_t)val);