This is a proposal to reduce of the size of the kbd-buffer root.
Currently the kdb-buffer is an area of ~250kb that is scanned
ambiguously.

With the patch, instead of tracing the entire kbd_buffer, we only scan
the region from kbd_fetch_ptr - 1 to kbd_store_ptr + 1.  The -1/+1 is
supposed to cover the cases where MPS stops the mutator while those
pointers are being updated.  If the kbd_buffer is empty, then only 180
bytes are scanned.  It's still scanned ambiguously.