raid1: copy write hint from master bio to behind bio

Signed-off-by: Mariusz Dabrowski <mariusz.dabrowski@intel.com>
Reviewed-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Reviewed-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>