bfs: Fixed missing block allocator uninit in case of error

* If Volume::Mount() failed opening the root node, the Volume would be
  destroyed regardless of the fact that the block allocator was
  initializing the bitmap asynchronously.
* For a proper shutdown, the block allocator needs to be uninitialized
  first.
* This should fix #15015.

Change-Id: Ice520a12c5c58c785f391009327becfb1f284bce
Reviewed-on: https://review.haiku-os.org/c/1501
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

src/add-ons/kernel/file_systems/bfs/Volume.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright 2001-2017, Axel Dörfler, axeld@pinc-software.de.
+ * Copyright 2001-2019, Axel Dörfler, axeld@pinc-software.de.
  * This file may be used under the terms of the MIT License.
  */
 
@@ -435,6 +435,9 @@ Volume::Mount(const char* deviceName, uint32 flags)
 	} else {
 		status = B_BAD_VALUE;
 		FATAL(("could not create root node!\n"));
+
+		// We need to wait for the block allocator to finish
+		fBlockAllocator.Uninitialize();
 		return status;
 	}