xen/common: Add NUMA node id bounds check to page_alloc.c/node_to_scrub

xen/common: Add NUMA node id bounds check to page_alloc.c/node_to_scrub

When building for Power with CONFIG_DEBUG unset, a compiler error gets
raised inside page_alloc.c's node_to_scrub function:

common/page_alloc.c: In function 'node_to_scrub.part.0':
common/page_alloc.c:1217:29: error: array subscript 1 is above array
            bounds of 'long unsigned int[1]' [-Werror=array-bounds]
 1217 |         if ( node_need_scrub[node] )

It appears that this is a false positive, given that in practice
cycle_node should never return a node ID >= MAX_NUMNODES as long as the
architecture's node_online_map is properly defined and initialized, so
this additional bounds check is only to satisfy GCC.

Signed-off-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>