[POWERPC] Fix SLB initialization at boot time

This partially reverts edd0622b

.

It turns out that the part of that commit that aimed to ensure that we
created an SLB entry for the kernel stack on secondary CPUs when
starting the CPU didn't achieve its aim, and in fact caused a
regression, because get_paca()->kstack is not initialized at the point
where slb_initialize is called.

This therefore just reverts that part of that commit, while keeping
the change to slb_flush_and_rebolt, which is correct and necessary.

Signed-off-by: Paul Mackerras <paulus@samba.org>