Fix "eject when unmounting".

It seems getting the device from the partition after it is unmounted
does not work reliably. Get the device much earlier in the function,
before actually unmounting it.

Fixes #2630.

src/servers/mount/AutoMounter.cpp

@@ -616,11 +616,15 @@ void
 AutoMounter::_UnmountAndEjectVolume(BPartition* partition, BPath& mountPoint,
 	const char* name)
 {
-	BDiskDevice device;
+	BDiskDevice deviceStorage;
+	BDiskDevice* device;
 	if (partition == NULL) {
 		// Try to retrieve partition
 		BDiskDeviceRoster().FindPartitionByMountPoint(mountPoint.Path(),
-			&device, &partition);
+			&deviceStorage, &partition);
+			device = &deviceStorage;
+	} else {
+		device = partition->Device();
 	}
 
 	status_t status;
@@ -678,10 +682,10 @@ AutoMounter::_UnmountAndEjectVolume(BPartition* partition, BPath& mountPoint,
 			bool	fHasMounted;
 		} visitor;
 
-		partition->Device()->VisitEachDescendant(&visitor);
+		device->VisitEachDescendant(&visitor);
 
 		if (!visitor.HasMountedPartitions())
-			partition->Device()->Eject();
+			device->Eject();
 	}
 
 	// remove the directory if it's a directory in rootfs