Since background water mark reclaim is scheduled by workqueue, it could
do more work than direct reclaim, i.e. write out dirty page, etc.

So, add PF_KSWAPD flag, so that current_is_kswapd() would return true
for memcg background reclaim.  The condition "current_is_kswapd() &&
!global_reclaim(sc)" is good enough to tell current is global kswapd or
memcg background reclaim.

And, kswapd is not allowed to break memory.low protection for now, memcg
kswapd should not break it either.
Reviewed-by: NGavin Shan <shan.gavin@linux.alibaba.com>
Reviewed-by: NXunlei Pang <xlpang@linux.alibaba.com>
Signed-off-by: NYang Shi <yang.shi@linux.alibaba.com>