If multiple threads call FsVolumeList#checkDirs at the same time, we should only do checkDirs once and give the results to all waiting threads

checkDirs is called when we encounter certain I/O errors. It's rare to get just a single I/O error... normally you start getting many errors when a disk is going bad. For this reason, we shouldn't start a new checkDirs scan for each error. Instead, if multiple threads call FsVolumeList#checkDirs at around the same time, we should only do checkDirs once and give the results to all the waiting threads.