page_cgroup: reduce allocation overhead for page_cgroup array for CONFIG_SPARSEMEM

Currently we are allocating a single page_cgroup array per memory section
(stored in mem_section->base) when CONFIG_SPARSEMEM is selected.  This is
correct but memory inefficient solution because the allocated memory
(unless we fall back to vmalloc) is not kmalloc friendly:

        - 32b - 16384 entries (20B per entry) fit into 327680B so the
          524288B slab cache is used
        - 32b with PAE - 131072 entries with 2621440B fit into 4194304B
        - 64b - 32768 entries (40B per entry) fit into 2097152 cache

This is ~37% wasted space per memory section and it sumps up for the whole
memory.  On a x86_64 machine it is something like 6MB per 1GB of RAM.

We can reduce the internal fragmentation by using alloc_pages_exact which
allocates PAGE_SIZE aligned blocks so we will get down to <4kB wasted
memory per section which is much better.

We still need a fallback to vmalloc because we have no guarantees that we
will have a continuous memory of that size (order-10) later on during the
hotplug events.

[hannes@cmpxchg.org: do not define unused free_page_cgroup() without memory hotplug]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>