Avoid touching FUSE mounts on vold binder threads

On user unlock when persist.sys.fuse property is set,
StorageManagerService calls into vold to link the primary
volumes. Because this involves accessing a FUSE path that
has not been initialized, vold should offload this work
from the binder thread, otherwise it would wedge and the
system server would wedge causing a Watchdog trigger.

This fixes a bug where we 'link primary' twice and vold
gets wedged on system server restarts.

Bug: 140064376
Test: with the FUSE property set:
adb shell stop && adb shell start && adb shell ls /sdcard

Change-Id: I0eb86f8ba256c385c916e2a0389a4f7482fc3775

VolumeManager.cpp

@@ -395,6 +395,7 @@ int VolumeManager::linkPrimary(userid_t userId) {
 
         auto symlinkInfo = new std::pair<std::string, std::string>(source, target);
         std::thread(symlinkPrimary, symlinkInfo).detach();
+        return 0;
     }
 
     std::string source(mPrimary->getPath());