powerpc/64s: Fix unrecoverable MCE calling async handler from NMI

In the Linux kernel, the following vulnerability has been resolved:

powerpc/64s: Fix unrecoverable MCE calling async handler from NMI

The machine check handler is not considered NMI on 64s. The early

handler is the true NMI handler, and then it schedules the

machine_check_exception handler to run when interrupts are enabled.

This works fine except the case of an unrecoverable MCE, where the true

NMI is taken when MSR[RI] is clear, it can not recover, so it calls

machine_check_exception directly so something might be done about it.

Calling an async handler from NMI context can result in irq state and

other things getting corrupted. This can also trigger the BUG at

arch/powerpc/include/asm/interrupt.h:168

BUG_ON(!arch_irq_disabled_regs(regs) && !(regs->msr & MSR_EE));

Fix this by making an _async version of the handler which is called

in the normal case, and a NMI version that is called for unrecoverable

interrupts.