ext3: Replace 0 with NULL for pointer in super.c file

Fixes the following sparse warning:
fs/ext3/super.c:983:45: warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext3/super.c

@@ -975,7 +975,7 @@ 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);
 		switch (token) {
 		case Opt_bsd_df: