FSUtils#waitOnSafeMode can incorrectly loop on standby NN

We encountered an issue where HMaster failed to start with an active NN not in safe mode and a standby NN in safemode. The relevant lines in FSUtils.java show the issue:

    while (dfs.setSafeMode(org.apache.hadoop.hdfs.protocol.FSConstants.SafeModeAction.SAFEMODE_GET)) {

This call skips the normal client failover from the standby to active NN, so it will loop polling the standby NN if it unfortunately talks to the standby first.