Revert "Kernel: Make DoubleBuffer use a KBuffer instead of kmalloc()ing"

This reverts commit 1cca5142af.

This appears to be causing intermittent triple-faults and I don't know
why yet, so I'll just revert it to keep the tree in decent shape.

Kernel/TTY/MasterPTY.cpp

@@ -81,7 +81,7 @@ bool MasterPTY::can_write_from_slave() const
 {
     if (m_closed)
         return true;
-    return m_buffer.space_for_writing();
+    return m_buffer.bytes_in_write_buffer() < 4096;
 }
 
 void MasterPTY::close()