fix TreeInode::doPrefetch crash

Summary:
We have some crashes on macOS and Linux in `TreeInode::doPrefetch` See this example on macOS:
https://fburl.com/scuba/osquery_crash_reports/nwpiy38w
and these on linux:
https://fburl.com/scuba/coredumper/gjno0jyf

 Looking at the code the ServerThreadPool here send run the futures and collect all the results with  `collectAll`.  The crash is happening inside this `thenValue`:
https://www.internalfb.com/code/fbsource/[8e01d2961164]/fbcode/eden/fs/inodes/TreeInode.cpp?lines=4696-4698

If one of the futures fails, the `collectAll` returns exception for that one, and finish the function while the rest of them continue to work which could be the cause of the crash in accessing invalid memory. See the crash logs here: https://www.internalfb.com/phabricator/paste/view/P1614967024?lines=21-37

Changing `collectAll` to `collectAllSafe` then it wait for all the futures to return, and then return exception if any of them fail. This could fix that crash.

Thanks to genevievehelsel for suggesting this fix and teaching me how `collectAll` and `collectAllSafe` works.

Reviewed By: MichaelCuevas

Differential Revision: D63467931

fbshipit-source-id: 3c05aab86b6bada7ab24c5693e55b3a4380560af

eden/fs/inodes/TreeInode.cpp

@@ -4707,7 +4707,7 @@ void TreeInode::doPrefetch(
           load.finish();
         }
 
-        return collectAll(std::move(inodeFutures))
+        return collectAllSafe(std::move(inodeFutures))
             .thenTry([lease = std::move(lease)](auto&&) {
               XLOG(DBG4) << "finished prefetch for "
                          << lease.getTreeInode()->getLogPath();