dm thin: fix pool feature parsing

Commit 787a996cb251e20 ("dm thin: add error_if_no_space feature")
mistakenly forgot to increase the number of feature args supported.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index d2328bb..726228b 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -1941,7 +1941,7 @@
 	const char *arg_name;
 
 	static struct dm_arg _args[] = {
-		{0, 3, "Invalid number of pool feature arguments"},
+		{0, 4, "Invalid number of pool feature arguments"},
 	};
 
 	/*