md: add proper write-congestion reporting to RAID1 and RAID10.

RAID1 and RAID10 handle write requests by queuing them for handling by
a separate thread.  This is because when a write-intent-bitmap is
active we might need to update the bitmap first, so it is good to
queue a lot of writes, then do one big bitmap update for them all.

However writeback request devices to appear to be congested after a
while so it can make some guesstimate of throughput.  The infinite
queue defeats that (note that RAID5 has already has a finite queue so
it doesn't suffer from this problem).

So impose a limit on the number of pending write requests.  By default
it is 1024 which seems to be generally suitable.  Make it configurable
via module option just in case someone finds a regression.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid1.h

@@ -46,6 +46,7 @@ struct r1conf {
 
 	/* queue pending writes to be submitted on unplug */
 	struct bio_list		pending_bio_list;
+	int			pending_count;
 
 	/* for use when syncing mirrors:
 	 * We don't allow both normal IO and resync/recovery IO at