thp: put_page: recheck PageHead after releasing the compound_lock

After releasing the compound_lock split_huge_page can still run and release the
page before put_page_testzero runs.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>