snapshot: no error when extending above max size

When lvm2 calculates the maximal usable COW size and crops the user
requested size to this value, don't return the error result from
the 'lvextend' operation.

We already apply the same logic when resizing thin-pool beyond
the supported maximal size.

FIXME: The return code error logic here is somewhat fuzzy.

lib/metadata/lv_manip.c

@@ -5586,6 +5586,7 @@ static int _lvresize_adjust_extents(struct logical_volume *lv,
 				seg_size = lp->extents - existing_logical_extents;	// Recalculate
 				if (lp->extents == existing_logical_extents) {
 					/* Signal that normal resizing is not required */
+					lp->size_changed = 1;
 					return 1;
 				}
 			}