[POWERPC] Fix potential duplicate entry in SLB shadow buffer

We were getting a duplicate entry in the SLB shadow buffer in
slb_flush_and_rebolt() if the kernel stack was in the same segment
as PAGE_OFFSET, which on POWER6 causes the hypervisor to terminate
the partition with an error.  This fixes it.

Also we were not creating an SLB entry (or an SLB shadow buffer
entry) for the kernel stack on secondary CPUs when starting the
CPU.  This isn't a major problem, since an appropriate entry will
be created on demand, but this fixes that also for consistency.

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