thp: remove unnecessary khugepaged_thread check

Now, khugepaged creation and cancel are completely serial under the
protection of khugepaged_mutex, it is impossible that many khugepaged
entities are running

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@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

@@ -2333,11 +2333,8 @@ static int khugepaged(void *none)
 	set_freezable();
 	set_user_nice(current, 19);
 
-	while (!kthread_should_stop()) {
-		VM_BUG_ON(khugepaged_thread != current);
+	while (!kthread_should_stop())
 		khugepaged_loop();
-		VM_BUG_ON(khugepaged_thread != current);
-	}
 
 	spin_lock(&khugepaged_mm_lock);
 	mm_slot = khugepaged_scan.mm_slot;