mm,compaction: serialize waitqueue_active() checks (for real)

mm,compaction: serialize waitqueue_active() checks (for real)

Andrea brought to my attention that the L->{L,S} guarantees are
completely bogus for this case.  I was looking at the diagram, from the
offending commit, when that _is_ the race, we had the load reordered
already.

What we need is at least S->L semantics, thus simply use
wq_has_sleeper() to serialize the call for good.

Link: http://lkml.kernel.org/r/20170914175313.GB811@linux-80c1.suse
Fixes: 46acef048a6 (mm,compaction: serialize waitqueue_active() checks)
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Andrea Parri <parri.andrea@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>