dm table: pass correct dev area size to device_area_is_valid

Incorrect device area lengths are being passed to device_area_is_valid().

The regression appeared in 2.6.31-rc1 through commit
754c5fc7ebb417b23601a6222a6005cc2e7f2913.

With the dm-stripe target, the size of the target (ti->len) was used
instead of the stripe_width (ti->len/#stripes).  An example of a
consequent incorrect error message is:

  device-mapper: table: 254:0: sdb too small for target

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index c70604a..6f0d90d 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -1453,7 +1453,7 @@
 
 	list_for_each_entry(pg, &m->priority_groups, list) {
 		list_for_each_entry(p, &pg->pgpaths, list) {
-			ret = fn(ti, p->path.dev, ti->begin, data);
+			ret = fn(ti, p->path.dev, ti->begin, ti->len, data);
 			if (ret)
 				goto out;
 		}