x86: Close race condition on MCA task queues at startup

[jtl]

Summary

Currently, the CMCI handler checks the cold variable to determine whether it can schedule a task. The task queue is setup as part of the mca_startup() function, which is run well after the configure_final() function sets the cold variable to 0. Therefore, if an MCA arrives in the window between configure_final() and mca_startup() running, the code could try to schedule a task on an unintitalized task queue.

Close the race by specifically checking whether the mca_startup() function has run.

[jhb]

Alternatively we could check some variable that this sysinit sets (like mca_tq) instead of cold, but this is fine.

[jtl]

Updating the diff to account for 8 years of bit rot.