quota: Acquire dqio_sem for reading in dquot_get_next_id()

dquot_get_next_id() needs dqio_sem only for reading to protect against
racing with modification of quota file structure.
Reviewed-by: NAndreas Dilger <adilger@dilger.ca>
Signed-off-by: NJan Kara <jack@suse.cz>

fs/quota/dquot.c

@@ -2079,9 +2079,9 @@ int dquot_get_next_id(struct super_block *sb, struct kqid *qid)
 		return -ESRCH;
 	if (!dqopt->ops[qid->type]->get_next_id)
 		return -ENOSYS;
-	down_write(&dqopt->dqio_sem);
+	down_read(&dqopt->dqio_sem);
 	err = dqopt->ops[qid->type]->get_next_id(sb, qid);
-	up_write(&dqopt->dqio_sem);
+	up_read(&dqopt->dqio_sem);
 	return err;
 }
 EXPORT_SYMBOL(dquot_get_next_id);