fadump: Invalidate the fadump registration during machine shutdown.

If dump is active during system reboot, shutdown or halt then invalidate
the fadump registration as it does not get invalidated automatically.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>