BFS: Write-lock the stream inode in BPlusTree::MakeEmpty().

I was experimenting with forcing the CheckVisitor to always rebuild all
indices, when I got a KDL from a Tracker process that I had running
a query in the background. The fault address was invalid (0x7b4493418),
and the stack trace was bfs_read_query -> GetNextMatching ->
TreeIterator::Traverse. That method read-locks inodes, and it looks
like query references to inodes are managed correctly.

This seems like the most plausible culprit for missing locks. I didn't
manage to reproduce the KDL, unfortunately, but hopefully this resolves
it nonetheless.

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

@@ -853,6 +853,7 @@ BPlusTree::MakeEmpty()
 {
 	// Put all nodes into the free list in order
 	Transaction transaction(fStream->GetVolume(), fStream->BlockNumber());
+	fStream->WriteLockInTransaction(transaction);
 
 	// Reset the header, and root node
 	CachedNode cached(this);