powerpc/kdump: Only save CPU state first time through the secondary CPU capture code

We might enter the secondary CPU capture code twice, eg if we have to
unstick some CPUs with a system reset. In this case we don't want to
overwrite the state on CPUs that had made it into the capture code OK,
so use the cpus_state_saved cpumask for that and make it local to
crash_ipi_callback.

For controlling progress now use atomic_t cpus_in_crash to count how
many CPUs have made it into the kdump code, and time_to_dump to tell
everyone it's time to dump.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>