util: Detect use-after-destroy in simple_mtx

util: Detect use-after-destroy in simple_mtx

This makes simple_mtx_destroy set the counter to an invalid canary
value and then makes lock/unlock assert that the value is legal.

That way, calling lock/unlock after destroy will assert fail,
rather than deadlocking or potentially even working.

This has caught real deadlocks in dEQP multithreaded tests (in st/mesa
shader variant zombie list handling), which have since been fixed.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>