The error indicates that mapreduce wants to be able to write to /.  you have the owner as hdfs with rwx, you have groups with r-x,  and others set to r-x. Since you added mapred to the groups membership earlier by adding it to supergroup and making supergroup the group for / it is the group level permissions that we will need to modify.   

To get it working you can do the following:

sudo -u hdfs hdfs dfs -chmod 775 /

this will change the permissions on / to drwxrwxr-x

as for why mapreduce is trying to write to / it may be that it's trying to create /user and /tmp that you have defined as the user space and the temporary space.  if you don't have those directories you could instead do the following:

sudo -u hdfs hdfs dfs -mkdir /user

sudo -u hdfs hdfs dfs -chown mapred:mapred /user

sudo -u hdfs hdfs dfs -mkdir /tmp

sudo -u hdfs hdfs dfs -chown mapred:mapred /tmp