Currently memtrace doesn't build if NUMA=n:

  In function ‘memtrace_alloc_node’:
  arch/powerpc/platforms/powernv/memtrace.c:134:6:
  error: the address of ‘contig_page_data’ will always evaluate as ‘true’
    if (!NODE_DATA(nid) || !node_spanned_pages(nid))
        ^

This is because for NUMA=n NODE_DATA(nid) points to an always
allocated structure, contig_page_data.

But even in the NUMA=y case memtrace_alloc_node() is only called for
online nodes, and we should always have a NODE_DATA() allocated for an
online node. So remove the (hopefully) overly paranoid check, which
also means we can build when NUMA=n.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>