ext4: replace plain integer with NULL in super.c

Fixes the following sparse warning:
fs/ext4/super.c:1672:45: warning: Using plain integer as NULL pointer
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -1673,7 +1673,7 @@ static int parse_options(char *options, struct super_block *sb,
 		 * Initialize args struct so we know whether arg was
 		 * found; some options take optional arguments.
 		 */
-		args[0].to = args[0].from = 0;
+		args[0].to = args[0].from = NULL;
 		token = match_token(p, tokens, args);
 		if (handle_mount_opt(sb, p, token, args, journal_devnum,
 				     journal_ioprio, is_remount) < 0)