This patch adds dq_dup_ref to struct dquot. Functions altering just usage of
quota take just this duplicated reference, inodes, quotactl() helpers take real
dq_count reference. dqput() blocks if there are some duplicated references and
put reference is last 'real one'. This way is assured that quota IO is not
done from functions altering quota usage (quota structure is written on last dqput()).