UserlandFS: Fix detection when a file system should be deleted.

This was a regression from hrev39870 where a private Referenceable class
was replaced with BReferenceable. Referenceable::RemoveReference()
returned a boolean to indicate whether the last reference was released
while BReferenceable::ReleaseReference() returns the previous reference
count.

A file system would therefore be deleted when any but the last reference
to it would be released (when multiple volumes of that file system were
mounted and one of them was unmounted) and leaked otherwise (when only
one volume of that file system was mounted and then unmounted).

Fixes #11718.

src/add-ons/kernel/file_systems/userlandfs/kernel_add_on/UserlandFS.cpp

@@ -164,7 +164,7 @@ UserlandFS::_UnregisterFileSystem(const char* name)
 		if (!fileSystemInitializer)
 			return B_BAD_VALUE;
 
-		deleteFS = fileSystemInitializer->ReleaseReference();
+		deleteFS = fileSystemInitializer->ReleaseReference() == 1;
 		if (deleteFS)
 			fFileSystems->Remove(name);
 	}