powerpc/32: Fix hardlockup on vmap stack overflow

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

powerpc/32: Fix hardlockup on vmap stack overflow

Since the commit c118c7303ad5 ("powerpc/32: Fix vmap stack - Do not

activate MMU before reading task struct") a vmap stack overflow

results in a hard lockup. This is because emergency_ctx is still

addressed with its virtual address allthough data MMU is not active

anymore at that time.

Fix it by using a physical address instead.