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 <[e-mail]>

drivers/md/dm-thin.c

···
 	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"},
 	};
 
 	/*