reiserfs: Fix quota mount option parsing

When jqfmt mount option is not specified on remount, we mistakenly clear
s_jquota_fmt value stored in superblock. Fix the problem.

CC: stable@kernel.org
CC: reiserfs-devel@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>

fs/reiserfs/super.c

@@ -1164,7 +1164,8 @@ static void handle_quota_files(struct super_block *s, char **qf_names,
 			kfree(REISERFS_SB(s)->s_qf_names[i]);
 		REISERFS_SB(s)->s_qf_names[i] = qf_names[i];
 	}
-	REISERFS_SB(s)->s_jquota_fmt = *qfmt;
+	if (*qfmt)
+		REISERFS_SB(s)->s_jquota_fmt = *qfmt;
 }
 #endif