md/raid1: clean up request counts properly in close_sync()

md/raid1: clean up request counts properly in close_sync()

If there are outstanding writes when close_sync is called,
the change to ->start_next_window might cause them to
decrement the wrong counter when they complete.  Fix this
by merging the two counters into the one that will be decremented.

Having an incorrect value in a counter can cause raise_barrier()
to hangs, so this is suitable for -stable.

Fixes: e392bfc1d3ad94b1774445ed8d2d5cc893c0d378
cc: stable@vger.kernel.org (v3.13+)
Signed-off-by: NeilBrown <neilb@suse.de>