Now that gigantic pages are dynamically allocatable, care must be taken to
ensure that p->first_page is valid before setting PageTail.

If this isn't done, then it is possible to race and have compound_head()
return NULL.
Signed-off-by: NDavid Rientjes <rientjes@google.com>
Acked-by: NDavidlohr Bueso <dave@stgolabs.net>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: NHillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>