powerpc: Bring all threads online prior to migration/hibernation

This patch brings online all threads which are present but not online
prior to migration/hibernation.  After migration/hibernation those
threads are taken back offline.

During migration/hibernation all online CPUs must call H_JOIN, this is
required by the hypervisor.  Without this patch, threads that are offline
(H_CEDE'd) will not be woken to make the H_JOIN call and the OS will be
deadlocked (all threads either JOIN'd or CEDE'd).

Cc: <stable@kernel.org>
Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>