r1_bio->start_next_window is not initialised in the READ
case, so allow_barrier may incorrectly decrement
   conf->current_window_requests
which can cause raise_barrier() to block forever.

Fixes: 79ef3a8a
cc: stable@vger.kernel.org (v3.13+)
Reported-by: NBrassow Jonathan <jbrassow@redhat.com>
Signed-off-by: NNeilBrown <neilb@suse.de>