raid5-ppl: fix handling flush requests

Add missing bio completion. Without this any flush request would hang.

Fixes: 1532d9e8 ("raid5-ppl: PPL support for disks with write-back cache enabled")
Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <sh.li@alibaba-inc.com>