powerpc: Add kdump support to Collaborative Memory Manager

When running Active Memory Sharing, the Collaborative Memory Manager (CMM)
may mark some pages as "loaned" with the hypervisor. Periodically, the
CMM will query the hypervisor for a loan request, which is a single signed
value. When kexec'ing into a kdump kernel, the CMM driver in the kdump
kernel is not aware of the pages the previous kernel had marked as "loaned",
so the hypervisor and the CMM driver are out of sync. Fix the CMM driver
to handle this scenario by ignoring requests to decrease the number of loaned
pages if we don't think we have any pages loaned. Pages that are marked as
"loaned" which are not in the balloon will automatically get switched to "active"
the next time we touch the page. This also fixes the case where totalram_pages
is smaller than min_mem_mb, which can occur during kdump.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>