memcg: fix init_page_cgroup nid with sparsemem

Commit 21a3c9646873 ("memcg: allocate memory cgroup structures in local
nodes") makes page_cgroup allocation as NUMA aware.  But that caused a
problem https://bugzilla.kernel.org/show_bug.cgi?id=36192.

The problem was getting a NID from invalid struct pages, which was not
initialized because it was out-of-node, out of [node_start_pfn,
node_end_pfn)

Now, with sparsemem, page_cgroup_init scans pfn from 0 to max_pfn.  But
this may scan a pfn which is not on any node and can access memmap which
is not initialized.

This makes page_cgroup_init() for SPARSEMEM node aware and remove a code
to get nid from page->flags.  (Then, we'll use valid NID always.)

[akpm@linux-foundation.org: try to fix up comments]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>