powerpc: Fix fatal SLB miss when restoring PPR

When restoring the PPR value, we incorrectly access the thread structure
at a time where MSR:RI is clear, which means we cannot recover from nested
faults. However the thread structure isn't covered by the "bolted" SLB
entries and thus accessing can fault.

This fixes it by splitting the code so that the PPR value is loaded into
a GPR before MSR:RI is cleared.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>