thp: khugepaged_prealloc_page() forgot to reset the page alloc indicator

If NUMA is enabled, the indicator is not reset if the previous page
request failed, ausing us to trigger the BUG_ON() in
khugepaged_alloc_page().

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1797,6 +1797,7 @@ static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
 			return false;
 
 		*wait = false;
+		*hpage = NULL;
 		khugepaged_alloc_sleep();
 	} else if (*hpage) {
 		put_page(*hpage);