memcg: bad page if page_cgroup when free

Replace free_hot_cold_page's VM_BUG_ON(page_get_page_cgroup(page)) by a "Bad
page state" and clear: most users don't have CONFIG_DEBUG_VM on, and if it
were set here, it'd likely cause corruption when the page is reused.

Don't use page_assign_page_cgroup to clear it: that should be private to
memcontrol.c, and always called with the lock taken; and memmap_init_zone
doesn't need it either - like page->mapping and other pointers throughout the
kernel, Linux assumes pointers in zeroed structures are NULL pointers.

Instead use page_reset_bad_cgroup, added to memcontrol.h for this only.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: David Rientjes <rientjes@google.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hirokazu Takahashi <taka@valinux.co.jp>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>