Kernel: Properly reset m_unveiled_paths on execve()

When a process executes another program, its unveil state is reset. For
this, we not only need to clear all nodes from m_unveiled_paths, but
also reset the metadata of m_unveiled_paths (the root node) itself.

This fixes the following bug:
1) A process unveils "/", then executes another program.
2) That other program also unveils some path.
3) "/" is now unveiled for the new program.

Kernel/Syscalls/execve.cpp

@@ -553,6 +553,7 @@ KResult Process::do_exec(NonnullRefPtr<FileDescription> main_program_description
 
     m_veil_state = VeilState::None;
     m_unveiled_paths.clear();
+    m_unveiled_paths.set_metadata({ "/", UnveilAccess::None, false });
 
     m_coredump_metadata.clear();