powerpc/numa: Limit possible nodes to within num_possible_nodes

MAX_NUMNODES is a theoretical maximum number of nodes thats is
supported by the kernel. Device tree properties exposes the number of
possible nodes on the current platform. The kernel would detected this
and would use it for most of its resource allocations. If the platform
now increases the nodes to over what was already exposed, then it may
lead to inconsistencies. Hence limit it to the already exposed nodes.

Suggested-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724105809.24733-1-srikar@linux.vnet.ibm.com