md/raid1: fix writebehind bio clone

After bio is submitted, we should not clone it as its bi_iter might be
invalid by driver. This is the case of behind_master_bio. In certain
situration, we could dispatch behind_master_bio immediately for the
first disk and then clone it for other disks.

https://bugzilla.kernel.org/show_bug.cgi?id=196383



Reported-and-tested-by: Markus <m4rkusxxl@web.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Fix: 841c1316(md: raid1: improve write behind)
Cc: stable@vger.kernel.org (4.12+)
Signed-off-by: Shaohua Li <shli@fb.com>