page-cgroup: trivial cleanup

Add forward declarations for struct pglist_data, mem_cgroup.

Remove __init, __meminit from function prototypes and inline functions.

Remove redundant inclusion of bit_spinlock.h.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>