mcpu: Start locking at correct level

Commit 8716b1db added a new lock level, LEVEL_NODE_ALLOC. It is ahead of
LEVEL_INSTANCE. The latter was hardcoded in mcpu to be locked right
after the BGL, effectively ignoring LEVEL_NODE_ALLOC.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>